"Autenticamente Mexicano" reads the sign above this roadside eatery, a tiny little place surrounded by the Guatemalan and Mexican districts of central Palm Beach County. There's hardly a gringo in sight within. (They probably wouldn't know how to mix and match the pineapple, guava, and tamarind sodas with the rich selections from the cooks' steam table, anyway.) But even the monolingual really need to know only one word here: burrito. The fresh flour tortillas are unbeatable -- supple, delicate, faintly sweet -- folded around fillings of chicken or wonderful grilled steak topped with lettuce, tomato, onion, and a piquant and subtle salsa verde. The lunch special of one burrito, one taco, and rice and beans is a steal at $4.99; a cellophane four-pack of gum -- spearmint, peppermint, cinnamon, and tutti-frutti -- makes dessert for two bits.