Grease a 9 by 13-inch pan. Combine the cheeses and sprinkle half of the cheese in the bottom of the pan. Saute the mushrooms, onion, and bell pepper in the butter until tender. Put this mixture over the cheese in the pan. Sprinkle the spinach over this layer. Sprinkle the rest of the cheese over the casserole. Cover and place in the refrigerator overnight.

The next morning preheat the oven to 350 degrees. I use my blender to blend the eggs, flour, and milk. Pour this over the casserole and bake uncovered for 45 minutes or until the eggs are set and browned. Let the casserole set for 10 minutes before cutting the casserole and serving.