Arrange broccoli in an even layer in an 11 x 7-inch baking dish coated with cooking spray; set aside.
Combine milk, flour, salt, and pepper in a large saucepan over medium-high heat; bring to a boil. Cook 1 minute or until thick, stirring constantly. Remove from heat. Add cheddar and cream cheeses; stir until smooth. Stir in mayonnaise and onion. Spoon cheese mixture evenly over broccoli.
Place panko in a small bowl. Drizzle with butter, and toss. Sprinkle breadcrumb mixture evenly over cheese mixture. Lightly spray breadcrumb layer with cooking spray. Bake at 375° for 25 minutes or until mixture begins to bubble and breadcrumbs brown.