Preheat oven to 375 degrees.

1. Arrange the broccoli in an even layer in an 11×7 inch baking dish coated with cooking spray; set aside.

2. Combine the heavy cream, flour, salt, and pepper in a large saucepan over medium-high heat, and bring to a boil. Cook 1 minute or until thickened, stirring constantly. Remove from heat.

3. Add the cheddar cheese and cream cheese, and stir until smooth. Stir in the mayonnaise, onion, and water chestnuts. Spread the cheese mixture evenly over the broccoli.

4. Put the Panko in a small bowl. Sprinkle with the melted butter and mix. Sprinkle the breadcrumb mixture evenly over the cheese mixture. Lightly spray the breadcrumb layer with cooking spray.

5. Bake at 375°F for 25 minutes or until the mixture starts to bubble and the breadcrumbs are golden brown.