01 - Preheat oven to 375°F. Grease a 9x13-inch casserole dish.
02 - Melt butter in a medium saucepan over medium heat. Whisk in flour and cook for 1 minute.
03 - Gradually whisk in milk, stirring constantly until smooth. Cook for 3 to 5 minutes until thickened.
04 - Remove from heat and stir in Dijon mustard, garlic powder, onion powder, nutmeg if using, salt, and pepper.
05 - In a large bowl, mix cooked chicken, turkey ham, and half of the Swiss cheese. Pour in sauce and fold gently.
06 - Spread the mixture evenly into the prepared dish. Top with remaining Swiss cheese and Parmesan cheese.
07 - Toss panko breadcrumbs with olive oil and sprinkle evenly over the casserole.
08 - Bake for 30 to 35 minutes until golden brown and bubbling.
09 - Allow casserole to rest for 5 minutes before serving.