Melt 3 tablespoons margarine in medium saucepan over medium heat. Stir in flour, salt and pepper. Cook until mixture is smooth and bubbly, stirring constantly. Gradually stir in broth and milk. Cook until mixture boils and thickens, stirring constantly. Set aside.
In ungreased 8-inch square (2-quart) baking dish, combine all casserole ingredients; mix well. Stir in white sauce; blend well. Cover tightly with foil; refrigerate at least 8 hours or overnight.
Heat oven to 350°F. In small bowl, combine topping ingredients; mix well. Uncover; sprinkle with topping. Bake uncovered for 45 to 55 minutes or until bubbly and golden brown.