Au Gratin Potatoes

Cut potatoes into thin slices. In a saucepan, cook onion in margarine until tender. Stir in flour, salt and pepper. Cook over low heat, stirring constantly. Remove from heat and stir in milk and 1 1/2 cups of cheese. Heat to boiling and boil for 1 minute. Place potatoes in ungreased 4-quart casserole pan and pour cheese sauce over potatoes.

Cook uncovered at 350° for 1 hour. Sprinkle remaining cheese on top and bake until melted. Yields 4 to 6 servings.