Au Gratin Potatoes

6 medium potatoes
1 medium onion -- chopped
1/4 cup margarine
1 tablespoon flour
1 teaspoon salt
1/4 teaspoon pepper
2 cups milk
2 cups cheddar cheese, shredded
1/4 cup dry breadcrumbs
paprika

Wash potatoes. Pare thinly. Cut into enough thin slices to measure about 4 cups. Cook and stir onion in margarine in 2-quart saucepan until onion is tender. Stir in flour, salt and pepper. Cook over low heat, stirring constantly, until mixture is bubbly; remove from heat. Stir in milk and 1-1/2 cups of the cheese. Heat to boiling, stirring constantly. Boil and stir 1 minute. Place potatoes in ungreased 1-1/2-quart casserole. Pour cheese sauce on potatoes. Cook uncovered in 325F degree oven 1 hour 20 minutes or in 375 degree oven 1 hour. Mix remaining cheese and the bread crumbs; sprinkle over potatoes. Sprinkle with paprika. Cook uncovered until top is brown and bubbly, 15 to 20 minutes longer.