EVERY day, sundown brings a steady dinner-time stream of cars into the dusty parking lots of countless little slapdash cafes along the US-Mexican border.At each spot from Brownsville, Texas, to Tijuana, Mexico, locals confide that their little piece of culinary heaven - plastic chairs, chipped china, fiery salsa and all - serves real Mexican food. In the evolution of the taco as the symbol of Mexican food, what these little joints serve lies somewhere deliciously between the simple tacos first eaten by Aztecs as convenient lunch-time finger food in the fields, and the hard-shell, chain-restaurant extravaganzas that explode on impact with teeth. Here in the center of the Imperial-Mexicali Valley, an agricultural oasis straddling the international border, La Hacienda Cafe is that kind of place: typical of the border-cafe phenomenon, but offering uni que food that customers can't forget. An old Richfield gas station just off the runway of the Imperial County Airport fueled cars at the front pumps, airplanes at the back, and farmhands at the Silver Spur bar inside: The little red-tile-roofed stucco building was a country hangout as early as the 1930s. But in 1952 the gas pumps were closed and Sonora, Mexico, native Raul Caro, his wife, one cook, and one waitress opened La Hacienda. "The gringos complained the chile salsa was too hot. They'd send the waitress back and say 'Give me some catsup, recalls Mr. Caro, with a grin. "But I haven't changed my recipe, and now they come in and say it's not hot enough. I got these people to eat chile [salsa]," he says, noting that in the taming of local tastes, his staff has grown from two to 25 full-time employees. Outwardly, there's little to suggest what's so special at La Hacienda, a low-slung building plastered inside with neon, an inflatable Batman, pinatas, collections of horseshoes and mugs and bullfight posters. But by 6 p.m. it is crowded with customers, mostly Anglos. Cafes like La Hacienda become "Cinderella-type legends in which human qualities are attributed" to the food and place by owners and customers alike, observes Mario Montano, a University of Minnesota folklorist who studies American foodways, and border food in particular. Often "the most extraordinary claims" are made about cafes, he says, most notably the "real" Mexican food claim. Mexico has 12 gastronomic regions, and even within the US, many different kinds of food are identified as "Mexican," he says . Enchiladas, for example, come rolled and separate in South Texas, stacked and flat in New Mexico, and casserole-like in California and Colorado. 'THE whole notion of 'Mexican' food is American, it is ethnically specific to the US because Mexicans don't call it 'Mexican' food," observes Arizona State University geographer Dan Arreola, who is compiling an atlas of Hispanic Americans. So while La Hacienda's menu seems ordinary, local legend has grown around its version of a taco, an enchilada, or a burrito. What's uncommon for Mexican food this side of the border is that not a bit of ground round is used at La Hacienda. Bottom-round roast is used for taco and burrito meat: After long stewing in huge caldrons of green and red chiles, onions, and spices, the meat is stringy and spicy. The soft corn tortillas for tacos are stuffed with the meat, pinned with a toothpick, deep-fried, and t hen topped with a little lettuce and cheese. The specialty that shows up on almost every order is the "special quesadilla." A cousin of the quesadilla (melted cheese in a flour tortilla), the special quesadilla is a six-inch, pie-dough crescent stuffed with Monterey Jack cheese and deep fried until it balloons into a puffy golden pillow. Caro claims that it was his late wife who originated the special quesadilla by figuring out just the right amount of shortening for the dough. But asking for the recipe is tantamount to stealing a trade secret, and Caro, a man of few words to begin with, gets tight-lipped. As if the 200 special quesadillas his cook makes every day appear by magic, he says, m not sure a recipe exists." The special quesadilla is associated with much of the folklore about La Hacienda. Tales abound of people who fly in from the coast 200 miles away to the local airport or drive miles out of their way just for La Hacienda food. Take, for instance, Dana Lyon: He's an America West airline pilot who grew up on La Hacienda food. When he first flew his future bride home to meet his family, the first order of business - even before meeting mom - was to take her for "real Mexican food." Never mind that she had grown up in Northern California with a Mexican maid perfectly capable of serving authentic Mexican food; she had never had a "special quesadilla."