Pour the butter into the bottom of a 13 x 9-inch baking dish coated with cooking spray; tilt dish to coat. Place spinach evenly in bottom of dish; sprinkle evenly with cheeses.
Lightly spoon flour into dry measuring cups; level with a knife. Combine flour and remaining ingredients in a medium bowl; stir with a whisk until blended. Pour milk mixture over cheese. Bake at 350° for 40 minutes or until lightly browned. Serve immediately.