Cantina Laredo, St Martin's Courtyard, 10 Upper St Martin's Lane, London WC2

Spoiler alert. This review contains details of the single worst dish the reviewer has yet encountered in her professional career. If you don't want to know the ending, or are reading this in the hope of finding a great new Mexican restaurant, look away now.

I didn't want to hate Cantina Laredo. Good Mexican food can be a wonderful thing, and the arrival in central London of an American institution serving "gourmet Mexican food" sounded like something to celebrate. This summer, there's a mini-boom of new Mexican openings, all promising a more authentic take on the cuisine than the assemblies of starch, cheese and ochre slurry that chain restaurants pass off as the real enchilada.

So trust me, I went to Cantina Laredo in the genuine hope of some vivid, authentic Mexican food. Founded in Dallas, the brand is already well established in the States. The London branch, operating under franchise, occupies a new development on the traffic-clogged edge of Covent Garden, just up the road from Stringfellows. Border country, in other words, patrolled by hungry travellers who have journeyed many miles by bridge and by tunnel in search of sustenance.

Shimmering next to the busy road in a haze of exhaust fumes, Cantina Laredo's unshaded pavement terrace offers a gruelling al fresco option. Inside, the room is an entirely featureless air-conditioned box, in the bland patrician style, all shiny wood and solid leather banquettes, that signifies upscale dining in suburban American malls.

The long and relatively conventional menu lists the usual enchiladas, fajitas and tacos, alongside obvious gringo interlopers like cranberry pecan salad, or tiger prawn flautas with Monterey Jack cheese. Desserts include "Mexican brownie" and "Mexican apple pie". Clearly this is authentic Mexican food as experienced north of the border.

Lunch started inoffensively, with a complimentary bowl of tortilla chips and a couple of fresh tasting salsas, then took a plunge with the signature margarita (£9.50), so sweet and bland it recalled some lime-flavoured variant on Sunny Delight. Our waiter, clearly well drilled in the American style of service, launched into his rehearsed spiel about all sauces being made from scratch, and fish being bought fresh every day, then malfunctioned badly by moving off after we'd ordered our starters. He was mortified when we called him back to complete the order. Clearly we weren't the ladies who lunch he'd taken us for, but a pair of hungry oil barons in disguise.

The sheer size of our starters, each vast bowl packed with redundant chopped lettuce and other bulking agents, confirmed that this wasn't going to be an exquisite fine dining experience. Quesadillas stuffed with gooey Oaxaca cheese, mushrooms and caramelised onions offered a flavour experience straight out of Pizza Express. Cilantro chicken salad, made with several kilos of grilled chicken, black beans and avocado, also failed to deliver any of the expected jolts of flavour.

And then it came. The worst thing I've eaten in my professional career. The menu flagged it up as a signature dish, clearly using the phrase in the same way criminologists do when talking about the signatures of serial killers. A whole poblano pepper, stuffed with ground beef, pork, almonds and raisins, entombed in clammy batter and deep-fried into a damp, school dinnerish horror. As the pepper leaked what moisture it had, the batter grew heavy and grey. Instead of something fresh and green, I was faced with an item that looked and tasted like it had been dug up on the Mexican Day of the Dead.

Vying for honours in the worst dish stakes, my friend's pork shanks in chipotle-wine sauce had, in apparent defiance of the laws of gastronomy, been left dry and tough by their alleged braising. Both dishes came with the same garnishes, which remained largely undisturbed when our dishes were removed from the table. "This is a place for hungover blokes," my friend gloomily observed. Which would have been fine, if Cantina Laredo didn't advertise itself as an upmarket option.

It just goes to show that you should never trust a restaurant that calls itself "gourmet", just as you should never believe someone who calls himself "eccentric". Certainly, it's the first purportedly upmarket restaurant I've been to that serves desserts on an iron skillet, anointed with brandy butter. We ended with what my friend described as "brownie fajitas", while That's What I Call Shite Hispanic Pop wailed on the music system, and the staff cleaned up around us.

To pay £50 a head for such a soulless experience was bad enough; for our waiter then to leave the bill open when it already included 12.5 per cent service was the final flourish of this Mexican Two-Fingered Wave.

Cantina Laredo, St Martin's Courtyard, 10 Upper St Martin's Lane, London WC2 (020-7420 0630)

Food
Ambience
Service

Around £50 a head including wine and service

Tipping policy: "Service charge is 12.5 per cent discretionary, of which 100 per cent goes to the staff; all tips go to the staff"

Side Orders: Top tacos

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