Crunchy Macaroni Casserole

Ingredients

• 7 ounces macaroni, cooked and drained
• 1 medium onion, chopped
• 2 tablespoons butter
• 1/4 cup flour
• 3 cups Cheddar cheese, shredded
• 1/2 teaspoon dry mustard
• 1 teaspoon salt
• 1/8 teaspoon pepper
• 2 cups milk
• 1 cup potato chips, crushed (reserve 1/4 cup for top)