Meanwhile, in 2-quart saucepan, heat margarine until melted. Stir in flour, salt, mustard and pepper. Cook over medium heat, stirring constantly with wire whisk, until smooth and bubbly. Stir in milk. Heat to boiling, stirring constantly. Boil and stir 1 minute; remove from heat. Stir in cheese until melted. Pour sauce over broccoli in casserole. Sprinkle with cereal mixture.