IF YOU'VE ever wondered what those big, bold, bananas are that you see occasionally in the supermarket, they are plantains. Not to be confused with plantain grass that pesky lawn weed that harasses us all, plantain, the fruit, is part of the banana family, Musa paradisiaca , and a supremely delicious food.

Actually a tree-like shrub, it is a spectacular sight on a banana plantation, growing to a height of 20 to 30 feet and having broad, fan-like leaves 5 to 18 feet long.

As a child growing up in New Orleans, I remember the plantain, as common a dish on our table as boiled corn or black-eyed peas. Yet it never ceased to remain a treat in our family.

While its flavor is similar to a banana, the plantain is richer, deeper, lustier and, at the same time, more subtle. Even one of my brothers who couldn't abide a banana, was crazy about plantains.

A native of India, the plantain is commonly cultivated throughout the tropics where it is prepared in a large variety of ways. It is boiled, used in soups, french fried like potato chips and often used as a potatoe substitute.

When I first came to live in Washington, the plantain was even more difficult to find than grits. But as more and more Spanish-speaking people migrated to Washington, area merchants found it profitable to answer the demand for the plantain.

Nutritionally, the plantain is an excellent source of potassium, a good source of Vitamin C, and a fair source of phosphorus. One 31/2-ounce portion contains 119 calories.

Two things to remember about plantains: they must be cooked to be palatable; they should be fully ripe before eating. As they ripen, they become tender, the sugar in the fruit develops, and the flavor heightens.

Buy them green if you must, but hold them until ripe. I place mine on a sunny window sill, rotating them until the skins begin to darken. When approximately half the skin is black, they are ready to eat.

I have forgotten them occasionally and suddenly found myself with 3 or 4 solid black plantains on my hands. Even then, they are edible, but the firm, pulpy texture has become mushy, and they are not as appealing as when just ripe enough. FRIED PLANTAIN 1 ripe plantain Butter or margarine for frying

Peel the plantain and cut into 3 sections of about 2 to 3 inches long. Slice each section into thin strips about a quarter of an inch thick. Fry slowly on each side in butter or margarine until fruit is a light, golden brown and tender when pierced with the point of a knife. Serve immediately. PLANTAIN FLAMBE

For a dramatic dessert, plantain flambe can be prepared right at the dinner table in a chafing dish or electric skillet. Start with warm fried plantain slices, arrange in chafing dish, sprinkle with sugar, lime juice, and about 2 ounces warm banana liqueur. Serve with vanilla ice cream for a really exotic dessert. STEWED PLANTAIN 1 plantain, fried as above 2 tablespoons brown sugar 1/2 cup water Dash lemon juice

Using fried plantain as a basis, sprinkle slices with brown sugar, add water and simmer for about 5 minutes in covered pan. Add a dash of lemon juice for a piquant flavor. CANDIED PLANTAIN 3 or 4 plantains, sliced as for frying 3/4 cup brown sugar 1/2 teaspoon grated lemon rind Juice of 1 lemon Dots of butter

Place plantains in shallow, buttered baking dish. Sprinkle with brown sugar, lemon rind and lemon juice. Dot with butter and bake uncovered for about 20 minutes at 300 degrees. A delicious juice will bubble around the fruit. Remove when slices are tender.