Any Mexican will tell you that there's no such thing as a Best Mexican Restaurant. Ex-pats from south of the border know that you go to one taqueria for your tripe soup, another for your mole, a third for your chile rellenos, and a fourth for your tacos barbacoa — depending on who's in the kitchen. At Taqueria Elvira, hidden in a half-empty shopping plaza behind Congress Avenue, the Osorio family knows how to make an eminently respectable burrito ($8.99 for one filled with beef tongue and served with refried beans and yellow rice), an excellent huevos a la mexicana ($4.99 with rice, beans, and tortillas), and a fine, smoke-infused taco barbacoa ($1.75). But the thing that's going to knock you flat with admiration and change the way you think about lunch forever is the quesadilla ($1.99 each). Forget everything you think you know about quesadillas — those limp, tasteless rounds served as "small plates" at fern bars, filled with Monterey Jack and swimming in bottled tomato salsa. These are not those. The Elvira quesadilla is a pillowy, oily, handmade tortilla, maybe three-quarters of an inch thick, folded over and sealed around homemade queso fresco (or shredded chicken or picadillo), then topped with more shredded queso and lettuce and served with a fruity, fiery green salsa. The texture of the tortillas is spongy and melting, like the lightest pancake. The cheese inside is a cross between freshly churned butter and artisanal mozzarella, and its effect is to induce involuntary moans. This quesadilla is ideally matched with a bottle of Mexican Victoria beer — a pilsnery, darkish brew — or a glass of horchata, a milky rice water sweetened with vanilla and cinnamon.