Step 1

Preheat oven to 375°.

Step 2

Arrange broccoli in an even layer in an 11 x 7-inch baking dish coated with cooking spray; set aside.

Step 3

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.

Step 4

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.