Fast-food restaurants are elevating the baked potato from side dish to main course. Consider Tater Junction and Le Souperb in downtown Washington, the Beef and Potato Exchange at Alurel Centre and The Prime Potato at Harborplace in Baltimore, where potatoes with a choice of toppings are the whole meal, a newly popular alternative to burger and fries.

You can make a similar reversal at home, turning the potato into the star and the meat or seafood into the garnish.

There are two kinds of potato connoisseurs -- those who like their potato jackets tender and those who prefer a bit of crunch. Rubbing with a little salad oil is a good way to soften any skin, including a potato's. wFor crispness, bake spuds dry or rub with a bit of salt. (Coarse kosher salt works particularly well.) Before baking, pierce the skins in several places with a fork or paring knife so that steam can escape. Medium potatoes baked at 400 degrees will take about 45 minutes.

Following are some of our favority maindish potato toppings. Most are so easy to concoct that we often provide a selection of two or three at one meal. TUNA FLORENTINE TOPPING (4 servings) 2 tablespoons olive oil 1 medium onion, minced 2 cloves garlic, minced 16-ounce can tomatoes, including juice 6 1/2-ounce water-packed tuna, drained 10-ounce package frozen spinach, thawed and drained 4 drops hot pepper sauce Salt and pepper, to taste

Heat oil in a large skillet, then saute onions and garlic until tender but not browned. Chop tomatoes and add them with their juice. Flake tuna and add to skillet along with spinach, hot pepper sauce and seasonings. Simmer about 15 to 20 minutes until most of the liquid has evaporated. Stir occasionally. Serve over baked potatoes. LOW-CAL HERBED DAIRY TOPPING (4 servings) 1 cup low-fat cottage cheese 1 cup low-fat plain yogurt, divided 2 tablespoons mayonnaise (low-cal, if desired) 2 tablespoons chopped fresh parsley (or 1 tablespoon dried) 3/4 teaspoon dried dill weed 1/4 teaspoon celery seed 1/2 teaspoon prepared mustard 2 scallions (green onions), finely chopped 1/4 teaspoon salt

Place cotttage cheese and 1/4 cup yogurt in blender or food processor and process until smooth and creamy. Fold in remaining 3/4 cup yogurt along with rest of ingredients. (Note: If you are using a food processor, you may chop the parsley and scallions in it. The bulk of the yogurt is folded into the mixture because processing liquifies it too much.)

Let the mixture rest in the refrigerator at least 1 hour to give the flavors a chance to mingle. Serve over baked potatoes. BROCCOLI WITH MORNAY SAUCE (4 servings) 3 1/2 cups broccoli, but in 1-inch pieces 4 tablespoons butter 4 tablespoons flour 1/8 teaspoon salt Dash pepper 2 cups milk 15 ounces (about 2 cups) sharp cheddar cheese, shredded

Boil or steam broccoli 10 to 15 minutes. Meanwhile, to make mornay sauce, melt butter over low heat in a heavy suacepan. Blend in flour and seasonings. Cook over low heat, stirring until mixture is smooth and bubbly. Remove from heat and stir in milk. Return to burner and cook, stirring constantly, until sauce thickens. Blend in shredded cheese and stir to combine well. When cheese is incorporated into sauce, drain broccoli and add. Serve over baked potatoes. ITALIAN-STYLE SAUCE (4 to 5 servings) 1 medium onion, chopped 1/2 green pepper, chopped 2 cloves garlic, chopped 1 to 2 tablespoons cooking oil 29-ounce can tomato sauce 12-ounce can tomato paste 1/4 cup water 1 bay leaf 1/4 teaspoon oregano 1 teaspoon basil 1/4 teaspoon marjoram Salt and pepper to taste 1 1/2 cups ricotta cheese

In a large saucepan saute onion, green pepper and garlic in oil until onion is tender. Add tomato sauce, tomato paste and water. Stir well to blend thoroughly. Add seasonings. Bring to a boil, then reduce heat and cook, covered, about 40 minutes. Stir occasionally. Remove from heat. Add dollops of ricotta cheese to tomato sauce. Stir until just blended. Serve over baked potatoes.