You may have passed its grubby exterior a million times, but until you pull open the sliding door of Juana and Justiniano Gomez's 18-year-old restaurant, you'd never realize how charming it is. Kaleidoscopic colors, walls smothered in paintings and masks, strings of gourds and tiny straw hats hanging from the ceiling, tables packed with Guatemalan families. Mrs. Gomez cooks fat little chuchitos (tamales) with the texture of whipped butter and tender pieces of chicken braised in tomato sauce; enchiladas guatemaltecas layered high with braised pork, beets, and lettuce; and soups whose deep broths are deceptively clear. Her specialty may be her pepián, chicken braised in a golden stew of ground seeds, spices, and cilantro.