01 - Preheat oven to 375°F. Grease a 9x13-inch casserole dish thoroughly.
02 - Melt butter in a medium saucepan over medium heat. Whisk in flour and cook for one minute to form a roux.
03 - Gradually whisk in milk, stirring constantly until smooth. Simmer for 3 to 5 minutes until sauce thickens.
04 - Remove saucepan from heat. Incorporate Dijon mustard, garlic powder, onion powder, nutmeg (if using), salt, and black pepper.
05 - In a large bowl, mix cooked chicken, diced turkey ham, and half of the Swiss cheese. Add sauce and blend gently.
06 - Transfer the mixture evenly into the prepared dish. Sprinkle with remaining Swiss cheese and Parmesan cheese on top.
07 - Toss panko breadcrumbs with olive oil in a small bowl. Evenly distribute over the cheese layer.
08 - Place casserole in preheated oven and bake for 30 to 35 minutes, until golden and bubbling.
09 - Allow casserole to rest for five minutes before serving to set properly.