cheesy ham and potatoe casserole

Ingredients

• 2 tablespoons onion chopped
• 1/4 cup margarine
• 1/4 cup flour
• 1 teaspoon salt
• 1/2 teaspoon dry mustard
• 1 dash pepper
• 1 1/2 cups milk
• 2 cups cheddar cheese, shredded
• 1/2 pound ham, sliced
• 6 cups potatoes