Somehow, the folks who own Zona Fresca, a clean, bright little taqueria serving Cal-Mex style burritos, chiles relleños, and quesadillas, have managed to achieve a feat that has stumped many a local superchef: How to turn out a delicious piece of fish, consistently, honestly, and cheaply. For their fish taco, a fresh white filet is dipped in beer batter, deep fried in canola oil, then wrapped in a double corn tortilla stuffed with marinated cabbage and sour cream dressing. You pay $2.50 for this perfect minimeal, carry it over to Zona Fresca's salsa bar, which offers an array of mild to fiery sauces (we're partial to the tart, green tomatillo), squeeze on a bit of lime, eat your taco in a half-dozen bites, then head back to the counter for round two. You can spend the good part of a day engaged in this ritual, several times a week, for months or even years before you'll think of looking elsewhere for your fish sandwich. Crunchy on the outside, moist, light, and steaming inside, the fish is perfectly foiled by the bright salsa and crunchy cabbage. Zona Fresca looks like a fast-food joint with its walk-up counter and plastic utensils. It also acts like a fast-food joint -- you can get in and out in a quarter of an hour if you have to. But the strong flavors and freshness of their vegetables, lean meats, and beans cooked without lard are healthy enough to make lunch a guilt-free zone.