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.