01 - Preheat oven to 350°F. Lightly grease a 9x13 inch baking dish.
02 - Melt 4 tbsp butter in a saucepan over medium heat. Whisk in flour and cook for 1 minute, stirring constantly to form a smooth roux.
03 - Gradually whisk in milk, stirring constantly until the sauce thickens (about 3-4 minutes). The sauce should coat the back of a spoon.
04 - Stir in garlic powder, onion powder, salt, and pepper. Remove from heat and set aside.
05 - In a large bowl, combine diced chicken, chopped turkey slices, and Swiss cheese. Mix thoroughly to distribute ingredients evenly.
06 - Pour the prepared sauce over the chicken mixture. Fold together until all components are well coated and combined.
07 - Transfer the mixture to the prepared baking dish, spreading it evenly into a uniform layer.
08 - Sprinkle mozzarella cheese evenly over the top of the casserole.
09 - In a small bowl, mix panko breadcrumbs with 2 tbsp melted butter until evenly coated.
10 - Sprinkle the buttered breadcrumbs evenly over the mozzarella layer.
11 - Bake for 30-35 minutes until the top is golden brown and the sauce is bubbly around the edges.
12 - Remove from oven and garnish with fresh parsley if desired. Allow to rest for 5 minutes before serving.