and the salt. Cook, stirring constantly, until mixture thickens and boils; reduce heat. Cover and simmer 10 minutes, stirring occasionally; remove from heat. Stir until smooth.Spread one-third of the mixture in casserole. Dot with 1 teaspoon of the margarine. Sprinkle with 1/3 cup Parmesan cheese. Repeat twice. Sprinkle with Swiss cheese. Bake uncovered 15 to 20 minutes or until hot and bubbly. 6 SERVINGS (ABOUT 3/4 CUP EACH); 125 CALORIES PER SERVING.