Preheat your oven to 375°F (190°C). Grease a 9x13 inch baking dish. Spread the cooked chicken in an even layer across the bottom, followed by the diced ham. Sprinkle half of the shredded Swiss cheese over the top.

In a medium saucepan over medium heat, melt the butter. Whisk in the flour and cook for one minute to create a roux. Slowly pour in the milk while whisking constantly to prevent lumps. Continue to cook, stirring frequently, until the sauce thickens (about 5-7 minutes). Remove from heat and stir in the Dijon mustard, garlic powder, onion powder, salt, and pepper.

Pour the prepared Dijon sauce evenly over the layers of chicken, ham, and cheese in the baking dish. Sprinkle the remaining Swiss cheese over the sauce.

In a small bowl, combine the Panko breadcrumbs with the melted butter. Stir until the breadcrumbs are evenly coated. Sprinkle the buttered breadcrumbs over the top of the casserole.

Bake for 25-30 minutes, or until the sauce is bubbly and the topping is golden brown and crispy. Let the casserole rest for 5-10 minutes before serving.