1. In 3-quart saucepan, melt butter over low heat. Remove 2 tablespoons and mix with 3/4 cup of the KELLOGG'S CORN FLAKES cereal. Set aside for topping.
2. To remaining butter in pan, stir in flour, salt and onion. Cook and stir about 1 minute. Add milk, stirring until smooth. Increase heat to medium and cook until mixture boils, stirring constantly. Remove from heat.
3. Add cheese to hot sauce, stirring until slightly melted. Stir in spinach, eggs and 3/4 cup cereal. Spread mixture in shallow 1-quart casserole dish. Sprinkle cereal topping on spinach mixture.
4. Bake at 350° F about 25 minutes or until thoroughly heated. Serve hot.