Heat margarine in 2-quart saucepan over medium heat. Cook onion and celery in margarine about 2 minutes. Stir in flour, pepper and mustard. Stir in milk and chicken broth. Heat to boiling over medium heat, stirring constantly. Boil and stir 1 minute. Stir in cheese. Heat over low heat, stirring occasionally, just until cheese is melted. Sprinkle with paprika if desired. 6 Servings (About 1 Cup Each); 355 Calories Per Serving.