The world is full of good cooks. You probably know a few: they're the ones who insist that making pasta from scratch is 'easy' (yeah, sure); the ones you never see when they throw a dinner party because they're in the kitchen weaving amber strands of spun sugar around an impossibly delicate confection that will later get stuck in your fillings. Good cooks are sometimes made, sometimes born and very occasionally discovered on programmes such as MasterChef, the BBC's long-running culinary talent show.

Thomasina Miers was one such cook and the kind of winner we love: very slightly wacky. She walked away with the coveted trophy and vowed: 'I want to conquer the Everest of food.' High hopes. But unlike other winners of the show, she didn't just disappear into the sunken soufflé of obscurity that awaits the ex-reality star. The good news is that she opened up her own restaurant in Central London. The bad news is that it's Mexican.

We all know about Mexican food; it's refried. It's nothing but beans, and then more beans. It's leftovers made good. But this particular restaurant has just won an award for being one of the best, cheapest eats in town and since this is a time of financial prudence for all of us, I decided to see if Ms Miers could make beans mean something to me.

Wahaca isn't just the sound you make when you put a hot potato in your mouth. Wahaca serves street food - the kind of food your average Mexican might chomp on dusty byways - although there it is unlikely to be served on the tasteful white rectangular plates you'll find here.

Whoever designed this restaurant deserves plaudits: the window is dressed with wooden crates filled with beer, cabbages, pineapples and, bizarrely, giant tins of Italian plum tomatoes. The basement room is reached by the kind of stone steps you find in state schools and lit by large Perspex squares coloured lime and azure. What look like silver-slatted garage doors give the place the feel of a rehearsal room for West Side Story, especially since insistent music blares out. But Wahaca is clearly popular as, though this was a late lunch, the place was full and there were an astonishingly large number of very smiley staff.

I was meeting the actress, who had already moaned that a single Mexican dish contained a woman's entire daily calorie allowance. (Mexico has the second highest obesity rate in the world.) With this rather off-putting detail in mind, we ordered mojitos and nibbled guacamole along with some of their 'lighter, healthier pork scratchings' (surely an oxymoron?).

The menu is divided into sections: street food, soup and salads, and platos fuertes - bigger plates. All included plenty of ingredients I hadn't banked on: such as salads, broad beans and fresh herbs. You can also opt for sustainable fish, or steak from the grill, but predictably the actress eschewed the healthier option and, guided by our helpful waiter, we ordered lots of little dishes of street food. They sound like exotic dances: quesadillas, tostadas and taquitos, filled with black beans and cheese, chorizo and potato, and chicken cooked in tomato, in manageable portions. There are lots of vegetable dishes, too. We ordered nopalitos tostadas because cactus isn't something you'd normally scoff for lunch. It didn't taste of anything very much, but if cactus were luscious then it would have become an international delicacy with celebrity chefs charging a premium for it. A pumpkin seed-sprinkled green salad was very good.

It's unlikely that anyone would have room for the churros y chocolate ('Our favourite! Mexican doughnuts with a rich chocolate sauce!') but we managed a delicious mango sorbet. We were drinking coffee when a candled cake was brought to a neighbouring table and one of the waitresses ordered the entire restaurant to sing 'Happy Birthday'. This fell flat: community singing, if it has to happen, should be spontaneous. And although the staff were very sweet and very helpful, at times they were just a little too helpful.

These are only minor quibbles because Wahaca is a bit of a find, serving delicious, innovative and competitively-priced food in a lively atmosphere. A place where you can eat Mexican without having to wear an elasticated waistband. Ms Miers is on to another winner; this is a restaurant that amounts to a whole lot more than a hill of beans.