TOMATILLO sauce is a lot more interesting than the traditional Mexican red sauce, and not just because it's green.

Raw, it replaces salsa ranchero as a dip with corn chips. A sensational snack can be concocted of layers of hot tortillas, thinly sliced avocado and thin strips of monterey jack, cheddar or cream cheese topped with raw tomatillo sauce as a salsa verde.

Cooked tomatillo sauce transforms mild foods such as poached chicken, veal, turkey breast, pork, shrimp, or sausages and eggs into savory and original specialties. It's delicious over huevos rancheros in place of the more traditional red sauce. The slight acidic yet sweet flavor of the tomatillos produces a distinctive yet delicate sauce.

Even simpler, slice tomatillos into a salad and they lend their own unique flavor--like that of apples or green tomatoes, depending on your taste buds.

In Mexico and Central America, where they have been grown for centuries and used as a folk remedy for diabetes, tomatillos may be called tomates verdes, though they are not tomatoes at all. Their flesh is more solid, more seedy and less juicy than tomato pulp. Botanically, the green, walnut-sized Physalis ixocarpa are members of the cape gooseberry family of ground cherries. And they are cousins to the husk-like orange flowers known as Chinese lanterns so popular in dried arrangements.

The Aztecs knew these fruits as miltomatl, and you may encounter them in Spanish as tomates de cascara, tomatitos verdes, or tomates de bolsas. But if all this name calling is thoroughly bewildering, forget it. There is nothing confusing about the vitamin A and vitamin C-rich fruit itself.

Although they usually are sold only in cans, fresh tomatillos are becoming more available. They are in season now and will remain so through the summer. Most tomatillos are shipped from Mexico, but recently they have begun to be commercially cultivated on a small scale in the United States.

When buying fresh tomatillos, choose small, firm berries with their papery brown husks intact. Fruits should be green, not yellow, which is the color they turn as they ripen. Store them with the husks on in a brown paper bag (not plastic) in the refrigerator, where they will keep for about two weeks.

Fresh tomatillos present a few surprises. If the veined, five-toothed calyx or cup-like dry husk is hard to remove from the sticky green fruit, a short soak in cold water will loosen it. The sticky chemical coating on the skin also must be washed off.

Tomatillos yield delicious sauces, raw or cooked (in Spanish, cruda or cocida). Chopped, uncooked fresh tomatillos are sometimes served as a dip. Cooked tomatillos, fresh or canned, can be chopped and prepared as a sauce that receives no further cooking and served as a topping for enchiladas, chilis, rellenos and tacos. Both versions are called salsa cruda.

To prepare raw tomatillo sauce, process chopped tomatillos in a food processor or blender with some onion, coriander and perhaps a little hot pepper for piquancy.

For cooked sauce, simmer whole tomatillos for about 15 minutes in water to cover (it's important not to pierce the skin or the seeds will spill out). The tomatillos will be tender and ready to use in a sauce.

Canned tomatillos, available in Latin American groceries, are packed in an acidic salt solution, which should be discarded. They won't make the tastiest salsa cruda, but they do make a simple and quite acceptable substitute for the fresh fruit in a cooked sauce, especially one that's jazzed up with onions, garlic, chilies and even lettuce. COOKED FRESH TOMATILLOS (Makes about 2 cups) 1 pound tomatillos Water to cover

Remove and discard brown husks from tomatillos. Wash under cold running water. Do not pierce skin. Place in a saucepan and add water to cover. Bring to a boil, reduce heat, and simmer until tomatillos are soft, but still whole, about 15 minutes. Turn occasionally for even cooking. Drain and place in the bowl of a blender or food processor. Add a small amount of cooking liquid and process until smooth. Use as the basis of cooked tomatillo sauce. Freezes well. MEXICAN GREEN TOMATO SAUCE (Makes about 1 1/4 cups) 2 serrano chilis or more to taste 1/4 onion, finely chopped 1 garlic clove, finely chopped 2 sprigs fresh coriander, optional Salt to taste 1 cup tomatillos, freshly cooked (about 1/2 pound) 1/3 cup water

1 teaspoon finely chopped fresh coriander for garnish

In a mortar and pestle or blender, grind together chilis, onion, garlic, coriander sprigs and salt until almost smooth. Add tomatillos and water and grind briefly. Sprinkle with chopped coriander and serve as a dip or a topping for tacos, enchiladas, chilis rellenos, or eggs. Freezes well. Adapted from "The Cuisines of Mexico," by Diana Kennedy MEXICAN CHICKEN WITH GREEN TOMATO SAUCE (Makes 4 servings) 2 whole chicken breasts, halved Salt to taste 2 ounces blanched almonds 1 small onion, chopped 2 cloves garlic, minced 1/2 cup fresh coriander 2 canned serrano chilis or fresh hot peppers, seeded and chopped 1 cup shredded lettuce 1 cup cooked fresh tomatillos 3 tablespoons butter or margarine Pepper to taste

Poach chicken in salted water to cover until just tender, about 15 minutes. Cool, covered in pan. Discard skin and remove meat from bones in small strips. In the bowl of a blender or food processor grind almonds. Add onion, garlic, coriander, chilis, lettuce, tomatillos, and 1/4 cup of chicken cooking liquid and process until smooth.

Melt butter in a skillet and blend in sauce. Season with salt and pepper to taste. Add chicken shreds, simmer, stirring, over low heat until chicken is heated through. Serve immediately. Sauce will thicken if kept in the refrigerator. To reheat, add additional chicken broth. May also be served with shredded pork, sausages, veal, shrimp or other poultry. GUATEMALAN SHRIMP WITH GREEN TOMATO SAUCE (4 servings) 2 cups cooked fresh tomatillos (about 1 pound) 3 cloves garlic, crushed 3 fresh hot peppers, seeded and chopped 1/2 cup vinegar 1/4 teaspoon ground allspice 1/4 teaspoon oregano Salt and pepper to taste 2 pounds shrimp, cleaned and cooked

Place tomatillos, garlic and peppers in a saucepan and cook until tomatillos disintegrate. Add vinegar, allspice, oregano, salt and pepper and heat thoroughly. Add shrimp and heat through. Serve immediately. Adapted from "The Book of Latin American Cooking," by Elizabeth Lambert Ortiz VEGETARIAN GREEN TOMATO TACOS (4 servings) 10 taco shells 16-ounce can refried beans 1/2 head iceberg lettuce, shredded 2 tomatoes, chopped 1 green pepper, chopped 1 onion, finely sliced 2 celery stalks, chopped 1 small cucumber, seeded and chopped 1/2 pound shredded cheddar or monterey jack cheese 1 1/4 cups green tomato sauce (recipe above)

Heat taco shells in oven according to package directions. Heat refried beans in a saucepan over low heat. Arrange lettuce, tomatoes, pepper, onion, celery, cucumber and cheese in a divided tray. To serve, set out taco shells, beans, pre-arranged tray and sauce. Each diner places a spoonful of refried beans in a taco shell, adds vegetables and beans to taste and top it with sauce.