Ham and Potato Casserole

3 tablespoons margarine

3 tablespoons flour

1/4 teaspoon pepper

2 1/2 cups milk

1 large onion -- chopped

5 cups potatoes -- cooked and sliced

2 cups ham -- cooked and diced

1/2 cup cheddar cheese -- shredded

Preheat oven to 400 degrees. Grease a 2-quart casserole. Melt margarine; stir in flour and pepper. Add milk and cook until thickened, stirring constantly. Remove from heat and add onion, potatoes, and ham. Pour into prepared pan. Top with cheddar cheese. Bake for 45 minutes.