Heat oven to 350º. Spray 1 1/2-quart casserole with nonstick cooking spray. Melt margarine in 1 1/2-quart saucepan over medium heat. Stir in flour, mustard, salt and pepper. Cook, stirring constantly, until margarine is absorbed. Gradually stir in milk. Heat to boiling, stirring constantly. Boil and stir 1 minute.