Arrange broccoli in an even layer in an 11 x 7" baking dish coated with cooking spray; set aside.
Combine milk, flour, salt, and pepper in a large saucepan over medium high heat. Bring boil. Cook 1 minute or until thick; stirring constantly. Remove from heat. Add cheddar cheese and cream cheese; stirrin until smooth. Stir in mayo, 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 and broccoli mixture. Lightly spray breadcrumb mixture with cooking spray. Bake for 25 minutes or until mixture begins to bubble and breadcrumbs brown.