01 - Preheat oven to 350°F. Lightly grease a 9x13 inch baking dish with cooking spray or butter.
02 - In a large saucepan over medium heat, melt 4 tablespoons butter. Whisk in flour and cook for 1 minute, stirring constantly to form a smooth paste.
03 - Gradually whisk in milk, stirring constantly to prevent lumps. Continue cooking for 3-4 minutes until sauce thickens and coats the back of a spoon.
04 - Stir in garlic powder, onion powder, salt, and black pepper. Remove saucepan from heat.
05 - In a large mixing bowl, combine diced chicken, chopped turkey slices, and shredded Swiss cheese.
06 - Pour the prepared sauce over the chicken mixture. Fold gently until all ingredients are evenly coated.
07 - Spread the mixture evenly into the prepared baking dish. Sprinkle mozzarella cheese over the top in an even layer.
08 - In a small bowl, mix panko breadcrumbs with 2 tablespoons melted butter. Sprinkle evenly over the casserole.
09 - Bake for 30-35 minutes until the topping is golden brown, the cheese is bubbly, and the casserole is hot throughout.
10 - Remove from oven and let rest for 5 minutes. Garnish with fresh chopped parsley before serving, if desired.