It's important to note that the Rochelle Canteen in Shoreditch is open to anyone who fancies a good lunch. But eating there does make you feel like you're in on a secret. To get there, you turn off Shoreditch High Street, and stroll past Leila's Deli, purveyors of good coffee and excellent cheese, until you reach Arnold Circus, home of the magnificent Boundary Estate. At this point, if you are a first timer, you will feel a little anxious, there being no hint of a sign, much less of a table. Do not despair. Walk confidently towards the Victorian school at the Circus's far side. In a brick wall is set a door. "BOYS", it says above it. You have arrived. Ring the bell, and the door will swing open. The restaurant is in the old bike shed. Personally, I recommend the samphire with brown shrimp.

The Rochelle Canteen began its life, as the name suggests, as a cafeteria: a dining room for the artists and creative types who rent space in the converted Rochelle School. It's a white rectangle, in essence, kitchen at one end, dining room – wipe-clean tables, vintage Ercol chairs, Shaker pegs on which dangle three battered sunhats – at the other. As a legacy of this, it has no licence, and is open only on weekday lunchtimes. But in every other way, it has evolved into a serious restaurant. "It's amazing, isn't it?" says its co-owner, Margot Henderson, with a slightly startled air. "It's a proper place. I mean… the kitchen is really strong." A pause for contemplation, and then: "Are you going to have pudding? I think you should. James [Ferguson, the Canteen's chef] is fantastic at tarts."

Henderson came here eight years ago – although she is vague about dates – with her business partner Melanie Arnold; mostly, they needed a base, not to mention a kitchen, for their catering company, Arnold & Henderson. So it still surprises her when she sees it as it is today, brisk and noisy, customers continuing to arrive as late as 3pm. Do not, though, mistake mild amazement for a lack of ambition. Henderson is warm, funny, clever, enthusiastic and slightly eccentric. But she is also, I think, highly competitive and quite tough. Ask her how she felt when her husband, Fergus Henderson, left the French House Dining Room, the tiny Soho restaurant where they cooked together, to set up St John and she will tell you, no messing. "Was I envious? Yeah! I was really envious. I went kind of mental. I thought: something's gone wrong here. I was rolling along and then suddenly, it was: there's no room for you, you're having a baby." Her voice rises at the continuing outrage of a crime committed more than 15 years ago.

Has she forgiven Fergus? Yes. But her magnanimity must have been made easier by her own victories in the years since: the acclaim the Rochelle Canteen has received; the swanky client list she and Mel have built (Alexander McQueen, the Gagosian Gallery, Zaha Hadid). Even better, she has now written a cookbook so brilliant it already feels like a classic to rival Fergus's Nose to Tail Eating. "Are you serious?" she asks. Apparently, it was agony to write. "I tried to give the money [the publisher's advance] back at one point. I shouldn't have worried. I should have just done it. But I just kept putting it off, and then I would lose the thread."

Unlike most new cookbooks, You're All Invited isn't desperately trying to take up a position. There are no earnest lectures about seasonality, no dreary outbreaks of solidarity with hard-pressed working women. Her introductions to each recipe are witty, but minimalist. "White food is so chic," she says simply, before telling you how to make brandade. You could say it's a life's work in 300 pages: a load of reliable things that are easy to make and delicious to eat from salt cod and potato bake (the first dish she ever made for Fergus), to cabbage and truffle spaghetti (the first dish he made for her), to a Turkish coffee cake that is unlike anything I've come across (cinnamon, coriander, wholemeal flour). Plus, the odd cocktail – because she and Fergus do love a drink.

When I say it's the result of a life's work, I mean it. Henderson's mother was a journalist who wrote a series of books about eating out in their native New Zealand, so Margot went to posh French restaurants – the kind, as she points out, that had carpets – from an early age. "I always adored them." Her mother, though, had also fallen in love with the teachings of Dr Gayelord Hauser, the American nutritionist, with the result that she had thrown out anything refined and white; her children endured cider vinegar and honey in place of cordial. "So, when I was 10, I started catering for my younger brothers. I said: they just can't have bran biscuits for their birthdays."

The first recipe she tried was the ginger crunch from the Edmonds Cookery Book, the New Zealand household bible.
And then there were the snails. "I was about eight or nine. We were studying France at school. So we took all the snails from the garden. We drowned them, and then we fried them up with breadcrumbs. I thought they were delicious. After that, I was always rustling up snails for my parents' dinner parties, which was quite funny in Wellington. Or anywhere. My children wouldn't cook snails. I think I just wanted to be different."

She went to university to read English, but lasted "about two seconds… I just hated the discussions. I got really annoyed with everyone, waffling on". She left, and worked in a Mexican cantina – a place emphatically without carpets – to save up and come to London. "I'd fallen in love [with a boy who'd come to England], and we were all obsessed by London and the Face magazine then." There followed stints in bedsit London and then Australia – and that was when she got really serious about cooking, following Stephanie Alexander, the great Australian cook, and working in a slick Sydney brasserie.
"Then I came back to England. I missed it, and I needed a man. Everyone in Australia was gay; I kept falling in love with gay men. That's a bit of a problem with me." She laughs, delightedly. "I had this rice ball business. I cooked 50 kilos of rice every day in my flat, and made them into balls with carrots and stuff, and then I deep-fried them. Basically, it was bad food for macrobiotics."

But then, in a single afternoon, everything changed. "I'd bought this new dress, and I was feeling quite good. When I walked past 192 [the Notting Hill restaurant name-checked in Bridget Jones's Diary], I thought: I want to work there. So I went in. They said I could start Monday."

This was the beginning of a career that left Henderson with a CV that pretty much tells the story of all that was hip and delicious in 90s London. 192 was followed by the First Floor Restaurant nearby (Peter Gordon, soon to open his Sugar Club, was a colleague), and the Quality Chop House and the Eagle in Clerkenwell. It was at the Eagle that she met Fergus. He came to one of their sprawling Sunday lunches. "I thought he was an incredibly sweet man. I wasn't used to that." Soon afterwards, they met to discuss the idea of opening a restaurant together (Fergus was working in the Globe, a Notting Hill nightclub frequented by Lucian Freud). "He was so lovely. He let me talk about coriander, and everything. It went really well, and by the end of the weekend we were a couple. Within a fortnight, he was telling me what we were going to eat at our wedding – a salt cod feast – and within a month, he'd asked me to marry him." Did they go with the salt cod in the end? "No, we made cassoulet, for 300 people."

In 1992, they opened the French House Dining Room, above the French House pub in Soho. She pulls a face. "But, of course, Fergus left quite quickly to open his big restaurant, so leading to years of bitterness, which I might just be coming out of." She stayed on; the French House, being so small, was a good restaurant for a woman who wanted to have children (they have three teenagers), and the team was close enough that they would tolerate the odd toddler. But it was hard, watching Fergus build his reputation – and she was so tired. "I was exhausted. If I could get through a night without crying, it was a miracle. It was a difficult situation. Half of me wanted to work, the other half wanted to be with the children. But Fergus also put up with a lot. You mustn't forget that I was completely in awe of Fergus. I was a groupie! I remember we had these quails. I was going to bone them all out – very efficiently, of course – and he said to me: oh, we'll cook them whole. The way he changed me was really exciting. I was going towards that way of cooking, but he tipped me over."

She didn't choose to leave the French House; she and Mel (who replaced Fergus as her business partner, and who has three children herself) were eventually given their marching orders by the landlord after several happy years. But still: what does her experience say about why there are still so few women chefs? "It's a difficult question. I mean, why are there not more women doing all sorts of things? Children, obviously. I think, too, that it's hard to battle away from doing pastry – not that there's anything wrong with pastry. And tweezers! Men love all that stuff. Maybe a hot, fiery kitchen, throwing out tons of food, full of men is intimidating. But I loved it when I was younger. Flirting with the boys all day long! It was great."

She and Fergus have had their troubles. He has Parkinson's disease, though things have improved wondrously since, six years ago, he underwent deep brain stimulation (wires were implanted in his brain, enabling certain electrical signals from it to be blocked, thus helping to control his tremors). "It is a miracle," she says. "When you think that, before, he was moving all the time. We went to a Japanese restaurant, and he couldn't get the food into his mouth. I said: 'Well, I could feed you. That would be quite sexy.' That was the only time he ever got cross. But that would never happen now." Fergus is adorably equable – or that is how he has always seemed to me. "Yes. He never complains. I envy that. I'm really moany. I've never heard him say: 'I'm sick of this.' I've never even heard him mention Parkinson's. He's incredible."

But you feel, too, that they are truly partners; her book is dedicated to Fergus, and he is one of the threads that run through it (the others being her mother, and her mother-in-law, excellent cooks, both). And now her children are growing up, the search is on for a site for a new restaurant. Does this mean the Rochelle Canteen will close? I feel, polishing off a quince and almond tart, slightly panicky at the thought. "Oh, no," she says. "We love it here. But it would be nice to have a place that was open for dinner and weekends. I mean, that's what normal restaurants do, isn't it? Dinner, weekends…" She hoots with laughter, though whether at the barminess of the present regime, or in expectation of the mayhem that lies ahead, I can't quite tell.