Step 1: Preheat oven to 350°F (175°C). Grease a 9×13 inch baking dish.
Step 2: In a large mixing bowl, combine the cooked chicken, diced ham, and 1 ½ cups of the shredded Swiss cheese.
Step 3: In a separate bowl, whisk together the cream of chicken soup, sour cream, milk, Dijon mustard, garlic powder, onion powder, salt, and pepper until the mixture is smooth.
Step 4: Pour the creamy soup mixture over the chicken and ham mixture. Stir gently to combine, ensuring that all ingredients are well coated with the sauce.
Step 5: Transfer the entire mixture into the prepared 9x13 inch baking dish. Spread it evenly to create a uniform layer.
Step 6: Sprinkle the remaining ½ cup of shredded Swiss cheese evenly over the top of the casserole.
Step 7: In a small bowl, combine the bread crumbs and melted butter (if using). Sprinkle this mixture evenly over the cheese layer for a golden, crispy topping.
Step 8: Bake for 25-30 minutes, or until the casserole is heated through, the cheese is melted and bubbly, and the topping is golden brown.
Step 9: Let the casserole cool for a few minutes before serving to allow the flavors to meld and the casserole to set slightly.