Meat is such a basic part of the American diet that many of us don't know how to cook without it. Yet it is possible to come up with interesting, tasty and nutritious main-dish recipes which do not require meat.

There are many advantages to learning something about vegetarian cooking. Nutritionists have been urging the American public to cut down on intake of red meat as a way to reduce the amount of saturated fat in our diets. What's more, relying on alternate sources of protein usually raises the roughage in your family's diet -- thought to be a possible protection against colon cancer. And cutting some of the meat from your shopping list can also help keep down grocery bills.

While few are interested in switching completely to a vegetarian diet, it is surprisingly easy to change your eating habits so that more of the protein comes from vegetarian and dairy sources.

In the early part of the century, Americans obtained more than half their daily protein from vegetable sources. Today most of us take less than a third of the necessary protein from non-animal sources.

Some of the non-meat foods which provide high concentrations of protein include soybeans, nuts, whole grains, peanuts, cereals, sunflower seeds, wheat germ, dried peas, sesame seeds, dried beans, soybean milk, cows milk, eggs and cheese.

However, unlike animal proteins, vegetable proteins do not contain the right combination of amino acids needed for protein to be completely utilized by the body. But the deficiencies in one vegetable protein can be overcome by eating them with complementary vegetable proteins or with dairy products.

The following is a list of foods containing complementary proteins:

Whole grains (brown rice, whole wheat, corn, etc.) with milk products; whole grains with legumes (soy beans, kidney beans, chick peas, etc.); legumes with seeds (sunflower, sesame, etc.); whole grains with seeds; seeds with milk products; legumes with milk products.

These recipes utilize complementary proteins.

TAMALE SOYBEAN PIE

(4 servings)

Filling: 1 or 2 tablespoons vegetable oil 1 medium onion, chopped 1 clove garlic, minced 1 can (16 ounces) tomatoes, including juice (or 2 large fresh tomatoes and 1/2 cup tomato sauce) 1/2 cup chopped green pepper 1/4 cup chopped ripe olives 2 teaspoons chili powder 1 teaspoon salt 1 can (15 ounces) soybeans (or 2 cups drained, cooked soybeans)

Crust: 3/4 cup stone-ground corn meal 2 cups milk 2 teaspoons vegetable oil 1/4 teaspoon salt 2 large eggs, beaten

Topping: 1/3 cup grated Monterey Jack or Cheddar cheese

First make filling: Heat oil in large skillet or saucepan. Saute onion and garlic a few minutes. Then add the rest of the filling ingredients except the beans. Simmer uncovered 10 to 15 minutes or until slightly thickened. Add the beans and continue to simmer while making crust.

For crust: Mix corn meal and 1/2 cup of the milk and set aside. Bring the remaining milk, oil and salt to a light boil. Then slowly add the corn meal mixture while stirring. Simmer continuously stirring, until thick. Then cover and cook over low heat 5 minutes more. Remove from heat and quickly stir in beaten eggs until they are well blended.

To assemble, put half of crust mixture in the bottom of a greased 6-cup casserole or baking dish. Cover with all the filling. Then top with the remaining crust. Sprinkle with the cheese. (Casserole can be refrigerated at this point and baked just before serving.) Bake at 375 degrees for about 40 to 60 minutes, using the longer time if the casserole has been refrigerated.

SPINACH CASSEROLE

(4 servings) 3/4 cup raw brown rice 1/2 cup shredded Cheddar cheese 2 eggs, beaten 2 tablespoons chopped parsley 1/2 teaspoon salt 1/4 teaspoon pepper 1 pound fresh spinach, washed and chopped 2 tablespoons wheat germ 1 tablespoon butter or margarine, melted

Cook rice according to package directions. Combine the eggs, parsley, salt and pepper. Combine the cooked rice and cheese. Stir the two mixtures together. Then add the raw spinach. Pour into a greased 10-cup casserole. Top with wheat germ which has been mixed with the melted butter. Bake in a 350-degree oven for 35 minutes.

FRESH VEGETABLE QUICHE

(4 servings as a main dish)

This recipe is especially easy because the "crust" is simply some slices of toasted bread. You can also use a partially baked pie shell (9 1/2-10 inches in diameter) -- preferably one made with some whole wheat flour -- instead. 4 or 5 slices whole wheat bread, toasted 2 tablespoons vegetable oil 1 onion, sliced 1 clove garlic, minced 5 cups mixed fresh vegetables cut in pieces (include broccoli, cauliflower, and carrots; mushrooms are optional) 1/4 cup sunflower seeds, unsalted (optional) 3 large eggs 1/2 cup ricotta (or cottage) cheese 1/2 teaspoon salt 1/2 teaspoon each thyme, marjoram, and basil 1/2 cup grated Swiss cheese

Grease an 8- or 9-inch square pan and line the bottom with the bread slices. Heat oil in a large saucepan. Saute onions and garlic until tender. Add rest of vegetables and sunflower seeds, if desired. Stir fry -- that is, stir continuously while lightly tossing -- for about 2 minutes. Add 2 tablespoons water. Cover and steam for 5 minutes.

Meanwhile, beat eggs in a small bowl. Then add ricotta and spices and mix well. When vegetables are ready, spread over bread slices. Pour egg mixture evenly over vegetables. Then sprinkle top with Swiss cheese. Bake 325 degrees for 35 to 40 minutes or until set. (This dish can be made in advance and refrigerated. Reheat in a 325-degree even until warmed through.)

BEAN, SPLIT PEA AND BARLEY SOUP

(8 to 10 servings) 12 cups water 3 packets vegetable broth 1 medium onion, chopped 2 ribs celery, chopped 2 carrots, sliced 1 1/2 cups dry split peas 1 1/4 cups great northern (or navy) beans 1/2 cup barley 1/4 teaspoon pepper 1/2 teaspoon salt 1/4 teaspoon garlic powder 1 bay leaf

Combine all ingredients in a large heavy pot. Bring to a boil. Cover and reduce heat. Cook at a very low boil for about 2 hours, stirring the bottom of the pot occasionally. Add additional salt and pepper if necessary before serving.

THREE-CHEESE LASAGNA

(6 servings) 1 medium onion, chopped 1/2 green pepper, chopped 2 garlic cloves, minced 2 tablespoons vegetable oil 2 cans (16 ounces each) tomatoes 1 can (6 ounces) tomato paste 1/4 cup fresh parsley, minced 1/2 teaspoon basil 1/2 teaspoon thyme 1 teaspoon osegano 1 bay leaf salt and pepper to taste 9 whole wheat lasagna noodles (available at health food stores or substitute regular lasagna noodles) 1 (10 ounce) package frozen chopped spinach, thawed and well-drained (optional) 1 pound ricotta cheese 1 egg 3/4 cup grated parmesan cheese 1/4 teaspoon pepper 8 ounces mozzarella cheese, grated

Saute onion, green pepper and garlic until tender. Stir in tomatoes, tomato paste and seasonings. Simmer uncovered 30 minutes. Mean-while, cook noodles according to package directions. Drain. In a separate bowl, combine spinach, ricotta, egg, 1/4 cup parmesan and 1/4 teaspoon pepper.

Spoon 1/4 of sauce into bottom of a 9-by-13-inch baking dish. Cover with 3 noodles, half of ricotta filling, 1/3 of the mozzarella cheese and 1/3 of the remaining parmesan. Repeat layer, using up ricotta filling. Cover with last 3 noodles. Top with remaining sauce and cheeses.

Bake at 350 degrees for 45 minutes. Let stand a few minutes before cutting into squares for serving.