Not to exaggerate or anything, but if you want my advice the key to happiness lies in the knowledge that the best lunch in Britain is served at Barrafina, a tiny tapas bar in Frith Street, Soho. OK, so it has only 23 covers and doesn't take bookings, a policy about which it is proudly strict. If I'm meeting a friend there, I aim to park my bum on one of its stools by midday at the latest and, once I am staring at the glistening pile of razor clams on ice, I find that I'm never anything less than starving hungry. What follows, then, is inevitably a feast: a few ham croquetas and pimientos de padron and chipirones (crisp fried baby squid); some morcilla (black pudding) with quail's eggs, perhaps a gooey tortilla, plus a plate of little gem with anchovy and pancetta; and, to finish, strawberries with Juanola sauce (Juanolas are pastilles made from unsweetened liquorice; you can buy them in Spanish pharmacies). Madhur Jaffrey told me that the tapas at Barrafina are better than any she has tasted even in Barcelona. I'd be lying if I said that I disagree.

Barrafina, which opened in 2007, is owned by Sam and Eddie Hart, the incredibly posh but delightfully amiable brothers who also brought us Fino. Its executive chef is Nieves Barragán Mohacho, who grew up near Bilbao, and who – or so the legend has it – could roast a chicken single-handed by the time she was seven. Needless to say, I have had a culinary crush on Nieves for years. It's not only that she makes such delicious spinach, pine-nut and raisin coca (a Mallorquin classic, coca is a pizza-like thing whose dough is made with manzanilla sherry). Nor is it the fact that she invented a tapa of tuna tartare which she serves with the most astonishingly smooth guacamole. No, tapas bars, hot and high-pressured, are often rather macho environments. It's wonderful to walk into one and see a woman – calm, quiet but indubitably commanding – running the show. If I visit, and she's not there, I always feel peculiarly disappointed.

So you can probably imagine how I felt when it was suggested that I might like to have a brief cooking lesson with her. I went completely over the top at first, and suggested that I work a whole shift by her side. Yes. During service. By the end of which time, I would be, or so I fantasised, Nieves's new right-hand woman. Needless to say, she didn't bite – and who can blame her? Following the recipes in the excellent new Barrafina cookbook is one thing. But turning out 30 or so plates of ham croquetas – the restaurant's most popular dish – in less than an hour is quite another. Why didn't I pitch up early one morning, before things got busy, instead? Nieves would, she promised, teach me how to make the croquetas, and how to use la plancha, the very hot plate on which pretty much anything can be grilled. This would be... confidence boosting.

In the days before this session, I hop around my own kitchen, trying hard to look simultaneously nonchalant and capable – a stretch, if I am honest, because I'm an incredibly anxious cook. In particular, I am weedy about hot oil, a yellow streak that is of no use whatsoever in the land of tapas, where so many things must be preternaturally crisp. Also, chopping. If Nieves is to shallots, garlic and parsley what a state-of-the-art combine harvester is to a field of wheat, then I am... basically a sickle. I would thrive in some kindly Shaker community, in a bonnet and a long pinny. But in her extremely narrow restaurant kitchen? When her mind is already turning to the ordered frenzy that is lunchtime? No. At this point, my plodding perfectionism will, I suspect, rapidly lose its charm.

The day itself dawns bright and sunny. I arrive at nine o'clock. Deliveries – a box of beautiful burnt sienna chanterelles, great bulging bags of hairy mussels – are arriving, vegetables are being peeled and chopped and boiled, the zinc bar is undergoing an assiduous clean. First things first, though. "Coffee?" says Nieves. Is she having one? "Of course!" She looks at me as though I am quite mad. "Always coffee," she says. "We need a LOT of coffee."

She gives me a chef's jacket, an apron, and a tea towel, which she folds and tucks into the knotted belt of my apron, in the professional way – at which point I am overwhelmed by a weird surge of energy: all this crisp linen is conspiring to make me feel just a tiny bit less of an imposter. "Ready?" she says. As I'll ever be.

First, la plancha, which is basically a smooth plate of chrome-plated steel, heated from beneath. The beauty of it is that the heat is evenly distributed (the heating element is circular, so warmth radiates outwards). It's also brilliantly non-stick. Nieves loves her plancha. "Life would be so good if you could have a plancha in the kitchen at home," she says. "It's so easy, and it's so versatile." First, she pours on a little olive oil. Then she takes a shiny sardine and flips it on. Two or three minutes pass. More oil. The fish is flipped over. Another two minutes pass. To find out if it is cooked all the way through, she prises it gently open with her fingers. This gesture, I think, is the defining characteristic of the professional chef as opposed to the home cook. The professional does not wimpishly poke and prod at things with a fish slice. The professional has asbestos fingers.

The fish is plated, and then… the real secret. Nieves reaches for her jug of ajillo, otherwise known as "magic" in the Barrafina kitchen. This sauce will lift almost any grilled dish, not to mention prettify on the plate, and here is how you make it: finely chop three garlic cloves and a handful of flat-leaf parsley, and then stir it into 75ml of good olive oil. Spoon over meat or fish sparingly. This all looks very easy and, in fact, it is easy. "You do one," says Nieves, and I do, using my fingers in the professional manner, squeezing my little fish ever so casually. "Perfect," Nieves says. And she sounds like she means it.

Next up, croquetas. Or at least, the bechamel filling that will be used in this evening's croquetas. Someone else will form my cold bechamel into a ball, dip it in egg, and roll it in breadcrumbs (at which point it will be swiftly fried). First I fry some chopped Serrano ham in oil for about 10 minutes. Then I add a load of flour, and cook for another 10 minutes. Then, slowly, I add warm milk. It's fine. Perfectly straightforward. But exhausting. Whisking restaurant-sized quantities of bechamel makes the arms ache, and the back, and the feet. Finally, I pour it into a tub, and cover it with clingfilm, so it does not develop a skin.

Outside in the sunshine, we eat our sardines (sod the early hour). Nieves tells me that it was her mother who taught her to cook: her grandmother was partially paralysed, which meant that her mum had to spend a lot of time at home; cooking was a way of keeping the little Nieves entertained. But she never dreamed it would lead to a career, let alone a career in London. Does she like it here? "I love Barrafina," she says, in a manner which suggests that, however proprietorial she feels about it, the restaurant will always have a life of its own (and it's true: the Harts are hoping to open a second branch soon). "I love the way I can see all of the customers, their faces, the way they feel about the food." She looks at me, her wannabe protegee, cheerily. "I'm happy to say that you aren't the only person who likes us." After this, she gets up, and heads back in the direction of her beloved plancha, and I go home, yellow streak eliminated, determined that all manner of miniature delights will soon be leaving my own kitchen, crisp and crunchy and amazingly delectable.