01 - Preheat oven to 350°F. Lightly grease a 9x13 inch baking dish with cooking spray or butter.
02 - Melt 4 tablespoons butter in a large saucepan over medium heat. Whisk in flour and cook for 1 minute, stirring constantly to form a smooth paste.
03 - Gradually whisk in whole milk, about 1/2 cup at a time, stirring continuously 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 - Transfer mixture to the prepared baking dish, spreading into an even layer.
08 - Sprinkle shredded mozzarella cheese evenly over the top of the casserole.
09 - In a small bowl, combine panko breadcrumbs with 2 tablespoons melted butter. Mix until crumbs are evenly coated.
10 - Sprinkle buttered breadcrumbs evenly over the mozzarella layer.
11 - Bake for 30-35 minutes until the top is golden brown and the sauce is bubbling around the edges.
12 - Remove from oven and let rest for 5 minutes. Garnish with fresh chopped parsley before serving.