For the uninitiated - picky wimps, the lot of them, if you ask me - there is something scary about Fergus Henderson's cooking. 'Eew!' they say, when you tell them that, for lunch, you ate pig's cap with dandelion, or blood cakes with fried eggs. 'No! You DIDN'T!' they squeal, when you admit that, for dinner, you scoffed tripe and onions, or giblet stew.

Occasionally, people wander into his restaurant, St John, in Smithfield in the City of London, knowing little or nothing about his menu. This is when things get interesting. A few wrinkle their noses and scuttle off in search of seared salmon, or the nearest outpost of Pizza Express. The rest knuckle down and order.

'They hover, looking anxious,' says Henderson, 41. 'They've been told we're good and they're wondering why. But then, as they eat, their whole aspect changes. That is incredibly rewarding. Our menu isn't designed to be a challenge. It's there to give pleasure. Yum, yum!'

Personally, Henderson can't see what all the fuss is about (it's all 'yum, yum!' to him), but it is interesting that his table-side manner, while certainly not in deference to the timid or the terrified, is none the less mild and coaxing. 'Come hither,' he seems to say, even as he waves a dripping organ in your face. Here he is, for instance, on the tricky issue of tripe: 'Do not let the word tripe deter you,' he writes in his classic - and now newly republished - book, Nose to Tail Eating. 'Let its soothing charms win you over and enjoy it as do those who always have!'

On the subject of warm pig's head, he urges: 'The flesh from a pig's head is flavoursome and tender. Consider, its cheeks have had just the right amount of exercise... and the nozzle has the lipsticking quality of being not quite flesh nor quite fat, the perfect foil to the crunch of the crispy ear.' And here, finally, are his thoughts on that rare treat, ducks' hearts on toast: 'The perfect snack for the cook who has just prepared five ducks. Have your toast ready.'

St John celebrates its tenth birthday this year - hence the lovely new hardback of Nose to Tail, first editions of which are now rumoured to sell for hundreds of pounds - and, in the decade since its doors swung open, it has become that rare and oxymoronic thing: a fashionable institution. On the one hand, there is something resolute and immutable about both its menu and its ethos; diners are served good, simple, seasonal food in a white room by helpful people in white aprons. On the other, it still cuts a certain dash, partly, I suppose, because its eclectic regulars include the art crowd (Sarah Lucas, Angus Fairhurst, Gavin Turk), the dancer Michael Clark, Bryan Ferry, Madonna (especially during her tweed phase), Cate Blanchett, even the editor of the Daily Mail. There is nothing showy about the place, but therein lies its charm. Impossible, really, to say whether it is super modern (the white walls, the terse menu), or super old-fashioned (the Eccles cakes, the Welsh rarebit). As its proprietor puts it: 'Restaurants should not be about moments.'

At the centre of this weird alchemy is Henderson himself, a man about whom no one - and I mean no one - has a bad word to say. The world of cooking is not generally noted for its bantam egos, nor for its supportive camaraderie; people have proteges, of course, but mostly they have rivals. Henderson, however, is adored by chefs and critics alike. Fay Maschler, restaurant critic of the London Evening Standard, says she could never love anyone who didn't love St John. AA Gill, her notoriously nasty colleague, is even more fulsome in his praise. He told me, and he sounded as if he meant it, that Henderson's cooking was 'properly moving' - that it could reduce him to tears. 'There's nothing magic about his food. It's not like the Fat Duck or Gordon [Ramsay], where you think: "how did this happen?" There are no smoke and mirrors. That is what makes it so special. Nothing is obscured. There's a clear line back to the farmyard. Any critic will tell you the same.'

But Henderson's supporter-in-chief is Anthony Bourdain, the chef at Les Halles in New York, self-styled culinary bad boy and author of the best-selling Kitchen Confidential. The story has it that Bourdain pitched up for dinner at St John, devoured a collection of bones - we shall return to these bones - and was so finger-tinglingly impressed that he ran into the kitchen and literally bowed down in front of Henderson.

'Every subsequent experience at the restaurant hit me like a percussion grenade,' he says in his foreword to the new edition of Nose to Tail Eating. 'I saw his simple, honest, traditional English country fare as a thumb in the eye to the Establishment, an outrageously timed head-butt to the growing hordes of politically correct, the Peta people, the European Union, the practitioners of arch, ironic fusion cuisine and all those chefs who were fussing about with tall, overly sculpted entrees of little substance and less soul.'

Does he have it right? Not exactly. As even Bourdain admits, Henderson is no ideologue, his book no manifesto. 'I'm quite sure, now that I've come to know him, that he in no way saw the unassuming food in this book to be an insult or an affront to anyone, much less a statement of any kind.'

Bourdain has been spreading the word with evangelical zeal ever since that first meal, with the result that his friend is now a red hot chilli pepper - albeit rather a diffident one - even in America, where offal (aka 'variety meats') is about as fashionable as Britney Spears. During a recent tour to promote his book in the US, Henderson was greeted by his culinary peers like Achilles come home from battle. At the legendary Chez Panisse, Alice Waters cooked a special dinner using his recipes. In Chicago, the noted chef Charlie Trotter halted service and sent his cooks into the dining room to clap Henderson to his table. In New York, there were celebratory dinners at Les Halles, Soho House and the Spotted Pig. Now that the Americans have finally 'got' Henderson, their passion verges on the competitive. On the dust jacket of the American edition of Nose to Tail Eating is a quote from Mario Batali, a Manhattan restaurateur. It says: 'Reading and dreaming of all these recipes makes me want to torch my own Babbo restaurant and move to London to heed the master's call.' Oh dear. They really have got it bad.

Fergus Henderson grew up in London, the son of two architects. His mother was a good cook - she came from Lancashire, so tripe and onions was a familiar dish even then - and her son had what he describes as 'an unhealthy interest' in cookbooks. After school, however, he, too, went off to train as an architect. It was during this period that he started cooking seriously, although he is vague about the precise details. 'I had a holiday job in a kitchen, but I think we'll draw a polite veil over that. There was nothing joyous or creative about it. And none of this helped my studies. After I left the Architectural Association, I worked in a few offices. The office lunch was a fairly earth-shattering moment, sitting eating at the drawing board. Lunch should be joy unbounding. My first proper kitchen was this funny little club that we set up in Mercer Street in Covent Garden. It got shut down. Then I worked at a club in Notting Hill. Then I met Margot.'

Margot, who is now his wife (they have three children), was also a cook. She had started at the Eagle, the famous Clerkenwell gastropub, and by this time was at the First Floor restaurant on Portobello Road. They got their first 'grown-up' kitchen together when they took over a room above the French House pub in Soho. 'I used to drink there rather frequently, so I knew the landlord. The good thing about it was that it involved as small a financial risk as possible. It was just the two of us and a kitchen porter. It was quite tiring, but then there were little moments of 'Ha ha! Hug and kiss and make-up'. So, you see, I've always had my own kitchen, and that's given me a kind of natural momentum. I've been allowed to follow my own pursuits. I should have sat at the feet of some wiser, older chef but, as it turned out, I didn't. I don't think it's done me too much harm.'

St John opened in October 1994. By this time, Henderson had met his business brain in the form of Trevor Gulliver, who had been behind the Fire Station in Waterloo. It was Gulliver who found their site. The building, a mere butcher's block from Smithfield meat market, was half Georgian townhouse and half old smokehouse, the last smoking having taken place in 1967. It had then been used, variously, as a place to grow beanshoots, a squat and the HQ of Marxism Today. Nowadays, the bar and bakery are in the old smoking room, with its 20-foot skylights and its opened-up towers, the restaurant in the old packaging room. Was he nervous? 'There was a certain amount of "this is it." Apparently, I didn't speak much when I got home. Fear helps in certain ways, but I was doing something that made me happy, cooking the food that I loved. That gives you confidence. We painted the room white; I was the only decoration.'

His reviews were almost uniformly brilliant, though he soon became tired of the puns that perched oh-so-wittily above them. 'The number that said: "offaly this" or "offaly that" or "ooh, you are offal, but we like you". It was sort of strange.'

Since then, he thinks, the menu has evolved, but only very subtly. 'I don't want to go into nursery-food territory. Sometimes, I'll wake up and I'll think: "Ha ha! Let's have..." But more often the menu is led by Ben the Fish ringing up and saying, "The boats have come in and this is what's on them." I find what one wants to eat and cook is so mood-led. One morning, you wake up and you want kippers; the next, you want fish pie. The most delicious thing really alters day to day.' But what's his favourite dish of all? 'The salad of tomatoes and little gem in my book is a simple thing but it saved my life once after I was overly indulgent. I love it. I owe my life to it. A pig's trotter is a fantastic thing. The first night of my honeymoon in Paris, my wife fell asleep in her steak tartare, so my trotter kept me company.' He pauses for a moment, forks a heap of brown shrimp and white cabbage into his mouth, and then adds dryly: 'It had been a very long day.'

Before I met Fergus Henderson, someone described him to me as being exactly like a butcher from an old Forties movie, and it is true. You can very easily imagine him saying: 'Good morning, Mrs Higginbottom. Is it pork or lamb chops for you today?' With his splendidly ruddy face, his round horn-rimmed spectacles and his kooky, old-fashioned way of speaking (he tells me, for instance, that my lunch - pig's cap, roast lamb with anchovy, baked cheesecake with marc (remains of grapes that have been pressed for wine-making) - is 'very steadying' when, of course, what he really means is that I am a greedy pig), he seems not of this world at all, like a character from Trumpton brought to life. It is impossible to imagine him raising his voice, which is, I assume, why St John is always fully staffed. He is, however, pinned ruthlessly down by the tremor in what he refers to as his 'slightly ropey left side', a symptom of his Parkinson's disease. The condition was diagnosed in 'around 1998 - I can't remember when, quite', and it has prevented him from working in his beloved kitchen for the past year.

He will hate me for saying this, but Henderson bears the burden of his illness lightly, bravely, with dignity and good humour - all of which makes you want to hug him. The tremor in his left arm is like a wave. It rolls up, bashes against the human shore for a while and then, for no reason at all, subsides again. But he just ignores it and, after a while, so do you. 'If I let it annoy me, it would be annoying,' he says. 'I can get a good old twitch up. But I don't let it get me down. Most of my chums are used to me. I'm here every day. I have a pretty full life. It's not like I'm all doom and gloom.' Did he feel gloomy when he first found out? 'I think when it was diagnosed, I thought: "Bugger." Then I probably had a good lunch and felt better about it. On the whole, I'm reasonably bonny. Possibly, I'm even a bit fitter now. I see a trainer, to help me even out the wobbles. He's a very wise chap.'

For the time being, however, there is no prospect of him going back into the kitchen in anything other than a supervisory role. 'It's not getting any better, put it that way.'

In October, depending on the vagaries of the NHS (there is only a 50 per cent chance that his name will be pulled out of the hat), Henderson is due to have deep brain stimulation, a revolutionary form of surgery that seems to help patients with their shakes. This involves the implantation of a wire with four electrodes at its tip into one of three target sites in the brain, and is done under local anaesthetic so that the patient's response can be monitored. The wire is then connected to a small unit under the skin in the chest which contains the equipment to generate electric signals for stimulation. The idea is that, post-surgery, the sufferer can switch his shakes on and off. (You may have watched a video diary, broadcast on Channel 4 recently, about former Guardian foreign correspondent David Beresford, who travelled to Grenoble to have the same treatment.)

'It'd be grand if the twitching could be reduced,' says Henderson. 'But it's not something I'm looking forward to. It's quite weird to make a decision to have someone stick two cables into your brain. The results are fantastic, and the surgeon is lovely. All very good. But one shouldn't think about it too much.' He drinks and smokes plenty because 'they advise you to do anything that makes you feel relaxed and happy'.

A little over a year ago, Henderson opened a second restaurant, St John Bread & Wine, along the road in Spitalfields, where you can eat breakfast and tea, as well as dinner. He claims this is it for the time being ('plans worry me') but I have a hunch that he likes the idea of opening a third St John, possibly beside the sea.

He is also writing a second book. 'It's peculiar. I'm downloading lots of baggage. It will be about everything from pickled walnuts to the importance of ritual.' He is all for the latter. He always has an elevenses of seed cake and madeira, and his favourite restaurant is Sweetings in the City. As he points out in Nose to Tail Eating: 'When having lunch at Sweetings, you sit at a bar behind which a waiter is trapped, you order your smoked eel, they yell to a runner who delivers your eel over your shoulder to the waiter, who then places it under the counter and then in front of you as if they had it all along. Not an entirely practical way of getting your food, but a splendid eating ritual.'

Apart from some new photographs, he has made no changes to the new edition of Nose to Tail. 'I felt that would be meddling with history. It marks what I was thinking about at that moment.' He tells me that the book was a 'small call' to people to befriend their butchers. Does he think things have improved since it was first published, or are the supermarkets marching relentlessly on? 'Well, in one sense, there's so much food in the media, cookbooks, people knowing things... but it's pornography. You look at it and you think you've done it, but you haven't. I think there's some hope. There does seem to be a growing number of fantastic farmers. But instead of that being the norm, it's a treat for restaurant kitchens.' He has never visited the Tesco Metro near his home in Covent Garden. 'The nightmarish nature of it. Everything is there, but in a strange, ersatz form. An avocado will be organic, but it's been flown in from Peru. It's all rather confusing.'

Perhaps you are wondering about those bones. Only one dish is always to be found on the menu at St John: roast bone marrow and parsley salad. This, and the fact that he occasionally serves squirrel, is the thing for which Henderson will always be best known; Bourdain has declared it his 'Death Row meal'. Do they sell? 'Oh, yes,' he says. 'Lots of bones.' The recipe, if you can call it that, is also included in Nose to Tail Eating. First, ask your butcher for some marrow bones from a calf's leg. Then, simply stick them in a hot oven for 20 minutes. Meanwhile, chop your parsley ('just enough to discipline it'), mix it with some shallots and a handful of capers (follow the 'Sultana Bran rule' when using capers - in other words, resist the temptation to add a few extra ones), and dress. Et voila! Just make sure you have a suitably long implement with which to scrape out the marrow so you can spread it on some toast. It sounds horrid, I know, but it really isn't. Bones, salad, toast, maybe a little salt. Life is all about such simple pleasures, something Henderson seems to understand better than any other living chef. 'Bones should be celebrated,' he says, smiling like a tipsy sphinx. 'The notion of gnawing on them is a thoroughly good thing.'

· St John Bar and Restaurant, 26 St John Street, London EC1 (020 7251 0848); St John Bread and Wine, 94-96 Commercial Street, London E1 (020 7247 8724)

Stuffed lamb's heart

serves 6

6 lambs' hearts (make sure they are intact, with a hole only at the top)

18 rashers of streaky bacon

1.1 litres chicken stock

stuffing

duck fat or butter

4 red onions, peeled and sliced

4 bulbs of garlic, peeled and chopped

2 large glasses red wine

225g yesterday's white bread, with crusts off, cubed

sea salt and pepper

half a bunch of sage, leaves only, chopped

First make the stuffing. In a pan with duck fat or butter cook your onions and garlic gently so that they do not colour but become soft and giving. Add the wine, let this reduce by half, then add the bread, season, and cook together gently for 15 minutes: if it appears too dry add a splash more wine. Cool then add the sage.

Meanwhile trim the hearts of any excess fat nodules at their openings and any obvious sinews, and the flap at the top. Finally, with your finger, scoop out any blood clots at the base of the ventricles. You are ready to stuff.

With your hand, press the stuffing into the heart, and level off the opening at the top. Then drape 3 rashers of bacon over the exposed stuffing in a star fashion forming a lid and secure with string.

Find an oven dish or deep roasting tray in which the hearts will fit snugly; stand them upright. Pour stock over - they do not need to be completely covered. Cover with tinfoil and place in a medium oven for 2? hours. When cooked remove and keep warm. Strain the juice and then reduce by half for a delicious sauce. Untie and serve with mashed swede.

Pea and pig's ear soup

serves 4

1.5 litres ham stock (preferably the water you boiled a ham in) or a ham bone plus a head of garlic

500g green dried split peas, soaked in water overnight and drained

2 pig's ears (ask your butcher, these should not be hard to obtain; singe off as much hair as you can)

2 whole white onions, peeled

sea salt and pepper

vegetable oil for frying

If you're using stock, bring it to the boil in a pan with the split peas, ears and onions, and then simmer until the peas are soft and cooked to a thick soupy consistency (approximately 3 hours). If it starts to get too thick add more stock or water. If you have a ham bone, just cover this with water, add your garlic, split peas, ears, and onion, and cook the same way as with stock, though it will probably need some skimming. Add more water if it is getting too thick. Season to taste. Remove the onions, and if you have taken that route the head of garlic and the ham bone.

Extract the ears from the soup, rinse them and dry them carefully. Allow them to cool and firm up, then slice very thinly. Heat vegetable oil in a deep frying pan (or deep fryer) and drop the ears in. Be careful, as they are likely to spit. When crispy remove from oil and lay on kitchen paper to drain off excess fat. Serve the soup hot. On top of each bowl place a cluster of crispy ear. If you have any boiled ham left you could incorporate small chunks.

Recipes taken from Nose to Tail Eating.

· Nose to Tail Eating is published by Bloomsbury, £16.99. To order for £14.99 call the Observer book service on 0870 836 0885