LAND OF PLANTAIN PLENTY. `Cooking banana' is a traditional main-dish staple in colonial Santo Domingo

IF you walk around the colonial center of Santo Domingo in the early hours of the morning, you will see men and women with hot pots of cooking oil or charcoal braziers. Most of them will probably be cooking plantains - a member of the banana family.

And when your breakfast is served to you there, the centerpiece is likely to be a plantain.

Breakfast in the Dominican Republic is anything but routine, especially if the visitor asks for some of the local foods and dishes, including not only the plentiful plantain but also local seafood.

For example, fishermen get up with the sun and search out oysters, which they carry along the beach in boxes. Those looking for something different for breakfast will find it here, fresh from the sea.

But plantains are the staple and traditional part of the diet here, and because they can be shipped long distances, it is also possible to cook and serve them far away from where they are grown.

A plantain is shaped like a banana, but is usually much larger. It's dark green, or green with brown blotches. The skin is hard and shiny. Ridges along the sides are pronounced. And the texture of the fruit inside is much like that of a potato.

If you go to the less-touristy hotels, you'll find plantains listed on the menu as mang'u, a traditional dish in the Dominican Republic, where this fruit has played an important part in the country's history.

Santo Domingo, the capital and a beautiful colonial city, was built on the strength of this food.

It was founded - twice - in a country that occupies half the island of Hispaniola in the Caribbean Sea, with Haiti on the other half. Bartolom'e Col'on - brother of Christopher Columbus - first founded the city in 1496. In 1502, Nicol'as de Ovando refounded it across the river. Santo Domingo is said to have a number of New World ``firsts.''

Its claims are:

The first European city.

The first street.

The first cathedral.

The first stone house.

The first sugar mill.

The explorations of Latin America began here, and Columbus established himself here, as did his brother and his son. Slaves who worked on the construction of the forts and palaces, in 1516, were fed a diet consisting mostly of bananas - which apparently sustained them. In time, many of the stone buildings fell to the ground.

In 1973, the government began to restore the historic buildings along the waterfront: the Tower of Homage, a fort, the two joined palaces called today the Casas Reales, now a museum, and others.

Plantains are not in the average North American's diet, but shoppers may see them in specialty supermarkets and wonder how to cook or serve them. Here is the way to make mang'u, the traditional breakfast of the Dominican Republic.

Mang'u (breakfast plantain) 1 very large plantain or 2 small ones (total 3/4 pound) 1 small onion 1 tablespoon oil 1 tablespoon vinegar 3 tablespoons butter Salt and pepper 2 eggs, fried, optional Peel plantain and cut into 2-inch sections. Boil in salted water 45 minutes or until the it is tender. Slice onion thinly across grain, separate into rings, saut'e in oil until limp.

Add salt, pepper to taste, and vinegar. Cook, stirring, 30 seconds. Drain lightly on paper towel. When plantains are cooked, drain and put in bowl with butter, and add salt and pepper to taste. With a fork, mash and mix until they resemble mashed potatoes.

Form into a mound and divide in half. Place each half of plantain on a separate plate and cover with half the onion rings. Add one fried egg on each, if desired, and serve.