Preheat oven to 375 degrees. Arrange broccoli in an even layer in an 11x7-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, onion, and water chestnuts. 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 degrees for 25 minutes or until mixture begins to bubble and breadcrumbs brown.