1. In 3-quart saucepan, melt margarine over low heat. Remove from heat. Remove 2 tablespoons margarine and mix with KELLOGG'S CORN FLAKES cereal. Set aside for topping.
2. To remaining margarine in pan, stir in flour, salt, pepper, sugar and onion powder. Gradually stir in sour cream. Fold in green beans. Pour into 10 x 6 x 2-inch (1 1/2-quart) glass baking dish coated with cooking spray. Sprinkle cheese and cereal mixture over casserole.
3. Bake at 400° F about 20 minutes or until thoroughly heated. Serve hot.