Restaurants, like fruit flies, make fine subjects for study. With each new generation, they not only teach us truths about their struggle for survival, but they also give us insights into our own behavior and how it shifts according to the times.

Witness Nopalito, a small, warm-blooded creature that seems perfectly adapted to an unforgiving climate. While older restaurants gasp for air in our post–asteroid strike economy, Nopalito, which was hatched amid the dust cloud, thrives. Everything about it is appropriately scaled, right down to its diminutive name. The Spanish word refers to a slice of cactus, though it also plays off Nopa, Nopalito’s parent restaurant, an upbeat outpost near the Panhandle with vibrant wood-fired cooking to match its lively mood.

In their new venture, co-owners Jeff Hanak and Allyson and Laurence Jossel have taken an even more casual tack. Their second restaurant sits just a few blocks from Nopa, next door to a supermarket and overlooking a parking lot. A covered patio doubles as an entryway into the dining room, a miniature space with blond-oak tables and avocado-colored walls. Hanging from the ceiling like an infant’s mobile is a collection of molinillos, the wooden, whisklike tools used for stirring hot chocolate. It’s a nice aesthetic touch, and it strengthens your suspicion: Nopalito deals in Mexican food.

On the one hand, a Mexican restaurant in San Francisco is no more novel than a corner nook in Brooklyn selling pizza by the slice. On the other, Nopalito so far exceeds the expectations of the genre that it nearly qualifies for a heading of its own. It’s not just the pedigree of its products—the organic pinquito beans, the pampered pork. Other Mexican restaurants can claim similar ingredients, even if their “slow food” isn’t always stuff you’re in a rush to eat. But Nopalito scores on two fronts: It combines our new First Lady’s insistence on finicky sourcing with the authentic depth of flavor that cheap-eats fiends attribute to their favorite taco trucks.

Credit for the cooking goes to José Ramos and Gonzalo Guzman, whose staff meals at Nopa were so impressive that their bosses thought the food deserved a public stage. The menu they’ve created is anything but fussy, yet the seeming simplicity of the dishes belies the effort that goes into them. The chefs and their staff perform the sort of tasks your abuela might have tackled: grinding their own masa, shaping their own tortillas, and seasoning their housemade green-and-red chorizo—which gets its colors from chilies and chard. Their mole is a rich, symphonic composition with six kinds of nuts and five types of chilies, pulverized and simmered in a virtuoso sauce. At lunch, the mole’s layered flavors infuse a shredded chicken enchilada. On the dinner menu, it provides intricate cover for a tender chicken leg.

It’s rare for an easygoing restaurant to show such painstaking exactitude. Lingcod, prepared delicately à la plancha, submits itself sweetly to spring onions and a smoldering salsa made from chilies de árbol. Tamales—blended with black beans, oyster mushrooms, and the corn truffles known as huitlacoche—have a lovely earthy heft, not the leaden weight of lard.

For every hearty dish, like birria de chivo, a redolent goat stew with chocolate, dried chilies, and roasted tomatoes, the kitchen turns out a sprightly, precise salad that could proudly show its face at restaurants twice as fancy. One is unabashedly bare-boned: confetti shreds of cabbage, carrots, and mandolin-sliced watermelon radish, dressed at the last minute in oregano and lime. Another makes artful use of grapefruit and blood orange slices arranged in overlapping starbursts, as bright and pretty as a fireworks display.

All of this comes wrapped in a package that’s slightly more expensive than a belly-bomb burrito but remarkably well priced. Nopalito’s great appeal—and what makes it a model of recessionary dining—is the way that it makes you feel like you’re on a special outing when nothing on the menu fetches more than 14 bucks.

The service is superb and enthusiastic. Your waiter really wants you to like him and his restaurant, and you really do, despite his irksome fondness for PC pronunciations of menu items. (You just want to eat out, not enroll in Berlitz.) In other words, there’s little reason to complain.

Nopalito also has a wine list, if you consider three a list. But the menu begs for beer and offers plenty of it, from a cold can of Tecate to a winning spin on the michelada, Mexico’s beer-based Bloody Mary, here upgraded with housemade tomato juice and zinged with chilies, salt, and lime.

Coffee isn’t served—perhaps by necessity at a restaurant that’s too little to encourage lingering. But the staff never make you feel unwelcome, and they give you a sweet sendoff: sugar-dusted almond cookies, delivered with the check. If that won’t suffice, there are also popsicles: seasonal fruit or dense, cinnamon-y chocolate. They’re worth savoring, but they’re portable, so you get the hint.

You step outside, reflecting on a small marvel of restaurant evolution. This may be where the dining scene is headed. And if that’s the case, our depleted atmosphere can still support a lot of life.

Nopalito: 306 Broderick St. (Bet. Oak and fell Sts.), S.F., 415-437-0303, wheelchair accessible, $$, three stars