CHICAGO — At a time when mole was still a one-syllable word, Rick Bayless asked Americans to discard nacho cheese notions of Mexican food and consider his experience of it — street food at once simple and complex.

Fresh from five years in Mexico, where the seasonal ingredients and nuanced flavors of traditional cooking had dazzled his palate, Bayless was confident American Mexican food could rise to that level of candid refinement.

Twenty years on, he's been proved right. Earlier this month, his Frontera Grill in Chicago — where straightforward style belies sophisticated, layered flavors — was named the nation's top restaurant by the James Beard Foundation.

For Bayless, it signifies more than recognition of his own cooking. It speaks to the democratization of American food, which he says for too long treated Mexican food as a "downscale cuisine."

"It tells you that food in America is not just one type of food; we embrace all kinds of cuisine," he said, sitting in his restaurant's eclectic bar the day after the award was announced.

"Because we are a melting pot of a country, we can fuse upscale dining with all sorts of ethnic cuisines in ways you just don't see elsewhere in the world," Bayless said.

Not that Frontera Grill is upscale, at least not by classic standards. Though sometimes gussied up for presentation, Bayless' cooking strays little from the Mexican market foods that first inspired him two decades ago.

"Our food is anything but stuffy," he said. "When you serve goat empanadas, stuffy doesn't work."

And Bayless' food certainly works. Such as his ceviche yucateco, a mix of steamed calamari and shrimp bathed in citrus. It begins with a wave of cilantro that quickly mellows into lime and orange with lingering habenero.

Or the Mexico City-style quesadilla, a delicate turnover and menu staple since Frontera Grill opened. Stuffed with locally made Jack cheese and peppered with epazote, a traditional Mexican herb, these are best described as sublime.

Where American Mexican cooking often assaults the senses, Bayless' hand with flavor and texture are both gentle and assertive. And his reliance on traditional technique and ingredients has offered an invaluable education.

"His was the first restaurant where you could really go and explore this cuisine and understand it's not just about a whole lot of chili pepper," said Ruth Reichl, editor in chief of Gourmet magazine.

Capturing the essence of traditional Mexican cooking is so important to Bayless, he takes his staff to Mexico every year to study regional cooking.

"Before Rick, we might have lumped all Mexican food together as if the same dishes were served coast-to-coast," said Dana Cowin, editor in chief of Food & Wine magazine.

"After Rick, we realized the broad spectrum of tastes and flavors," she said. "For example, we learned that every salsa isn't raw and made with tomatoes, nor is every mole made with chocolate."

The other arm guiding Bayless' cooking is sustainability. During summer, as much as 90 percent of his menu is locally sourced. And to make sure his chefs understand the food they work with, he sends them to work on his suppliers' farms.

In 2003, he created the nonprofit Frontera Farmer Foundation, which so far has given $300,000 to local farmers.

"Twenty years ago when we first opened, I started looking for local farms to work with and there were just none. All soy beans and corn," he said. "Twenty years later, there's just dozens and dozens."

But not all ingredients were hard to come by. Bayless moved to Chicago in part because of the city's large Mexican population. This gave him easy access to esoteric ingredients, as well as diners accustomed to traditional Mexican food.

And they quickly became accustomed to his brand of it, as well. Two years after opening Frontera Grill to great success, Bayless launched Topolobampo next door — an even more upscale Mexican restaurant.

Its success has convinced Bayless of the growing sophistication of the American appetite. More people have become willing to treat food — at every stage from ground to gourmet restaurant — as a craft, not a product.

When Bayless speaks of the behind-the-scenes forces that shape his food, it's easy to see the former anthropology doctoral student under his chef's coat. (He studied Latin American culture and linguistics before opening Frontera Grill.)

The latest award caps a series of recognition for Bayless, who authored numerous acclaimed cookbooks on Mexican food, as well as appeared in various public television cooking series.