'Would you like to try stone soup?" asked my guide as we drove east out of Oaxaca city along highway 190. I wasn't sure my teeth could take it, but she swung the car off the dusty road and pulled up beside an open-fronted wooden diner, where dozens of tables and benches waited in the shade.
The waiter delivered a jug of iced pineapple juice with a plate of freshly baked mushroom and pumpkin flower quesadillas, then led us to a smoky corner where the chef was poking around in a fire with long wooden tongs. Several gourd bowls filled with clear broth, sliced green chillis, coriander and onions, a few plump greyish prawns and a chunk of raw red snapper were lined up on a bench. When the stones glowed crimson with the heat, he pulled them from the flames with the tongs and dropped them into the bowls. The contents began to boil immediately, fizzing and spitting ferociously. By the time we had carried them back to our table the fish was cooked to perfection in its tangy spicy broth.
"We use river stones; they can only be heated once or they explode," said the chef. This pre-hispanic dish, caldo de piedra (stone soup), has been made by the Chinantec people of San Felipe Usila, in the state of Oaxaca, southern Mexico, for centuries. It is this sort of ancient dish that last month led Unesco to list Mexico's traditional cuisine (the only other is French gastronomy) as "intangible cultural heritage".
While Unesco's world heritage list recognises historic and natural wonders, the "intangibles" list covers traditions such as dances, songs, rituals and crafts – fragile links with the past, at risk of erosion not by weather or woodworm, but by modernisation and globalisation.
To those who know Mexican food as enchiladas, fajitas and burritos, this may be surprising. But the country's culinary heritage is several millennia old, with recipes from Aztec, Mayan and other groups fused with Spanish influences, and staples of corn, beans, avocado and chilli jazzed up with chocolate, cactus and grasshoppers.
One of the best regions for Mexican eating is Oaxaca, rich in indigenous culture and famous for its mole (chocolate sauce used in savoury dishes), and meat. Who could be a better guide to Mexican food than Oaxaca city's top chef, Alejandro Ruiz, whom I met the week after the Unesco announcement at the restaurant he runs in the city's luxurious Casa Oaxaca boutique hotel? As one of a new wave of contemporary chefs turning traditional cuisine on its head, he claimed not to give an ay caramba! for Unesco's seal of approval, though he does care about culinary history.
"I want to protect our ancient recipes. So many are lost when our old ladies die. But I also wanted to do something lighter, more modern."
Alejandro has travelled all over Mexico, gathering recipes and cooking techniques using unusual ingredients such as iguana, wild mussels and goat, striving for new interpretations of old recipes and methods.
El Restaurante (casaoaxacael restaurante.com), his main venture, has Oaxaca city's best views, and some of its best food. From a rooftop candlelit table, I drank a passionfruit mescaltini and watched teenagers canoodling on the polished steps of Santo Domingo cathedral. Cheese-stuffed pumpkin flowers (a local delicacy) were followed by duck tacos with frijole (bean sauce) and squishy suckling pig with white beans. Our only complaint was that it was so dark we couldn't see what we were eating, so much of the joy and presentation was lost.
Alejandro's favourite new restaurant is Pitiona (pitiona.com), opened three months ago by José Manuel Baños Rodríguez, a young chef with a stint at Spain's famous El Bulli restaurant under his belt. It shows. The food was incredible. My friend and I both ordered the six-course tasting menu (refusing the offer of a different accompanying mezcal with every course), but were each brought a different six. From the traditional sopa de fideos (noodle soup) with tiny floating cheese marbles – thin-skinned capsules of liquid which burst on the tongue – to the chicharrón (crisp pork skin) and prawn taco, the fish burger that looked like a scallop, the lamb with white bean spray-can mousse, all had serious wow factor.
But what about real traditional food, I had asked Alejandro, where do we go for that? His eyes lit up as he described the Sunday food market in the nearby village of Tlacolula, so the next day, we went. Amid the live turkeys, dried grasshoppers, basketball-sized cheeses and fermented cacao juice sold here by 22 indigenous communities, we found the barbacoa (barbecue) section. There women touted baked lamb and goat, thrusting out samples with their bare fingers. We picked the tastiest stall, called Chabelita, and enjoyed a tortilla of soft spicy meat cooked for five hours in a hole in the ground then rolled in lime juice, onions and radishes.
Further on were dozens of butcher's stalls and a line of charcoal barbecues; we bought paper-thin slices of beef shin and grilled them ourselves with spring onions and chillies.
While posh meals in the city were astounding, these traditional out-of-town places were the most exciting. Linda Hanna, the guide and B&B owner who had taken us for stone soup, introduced us to other traditional dishes at Azuzena Zapoteca (tilcajete.org), in the village of San Martin Tilcajete. Chiles en nogada was a fantastic patriotic dish of Mexican colours – green chillis stuffed with meat and dried fruit, with pomegranate seeds and creamy nut sauce.
Her last tip, revealed with a wink as she said goodbye, was to go to Hosteria de Alcalá (hosteriadealcala.com), back in the city, and to mention her name when asking for the café de diablo. We didn't know what to expect so were amazed when three waiters set up two tables beside us, lit grills, and set to work flambéing bananas, pouring several different liqueurs into a vat of coffee and, just before serving, setting a spiral of booze-soaked orange peel alight in a dramatic blaze of flames. Not exactly what Unesco had in mind, but a spectacular finale.
• Doubles at Casa Oaxaca (casaoaxaca.com) cost from £138 a night. Or stay at Las Bugambilias (i-escape.com, doubles from $80 a night), or out of town at Casa Linda (folkartfantasy.com, from $65 a night). Flights from Heathrow to Oaxaca via Mexico City in April and May 2011 start at £764 with British Airways and Aero Mexico, through Netflights.com (0844 493 4944, netflights.com). Further info: whc.unesco.org
