Prepare the Cream Sauce
Preheat your oven to 375°F (190°C). Grease a 9x13-inch baking dish.
In a medium saucepan, melt 4 tablespoons of butter over medium heat. Stir in the flour and whisk constantly for 1-2 minutes to create a roux.
Slowly add the milk and heavy cream, whisking until smooth.
Stir in Dijon mustard, garlic powder, onion powder, paprika, salt, and pepper. Continue cooking until the sauce thickens slightly, about 5-7 minutes. Remove from heat.
Assemble the Casserole
Layer the shredded chicken evenly in the prepared baking dish.
Sprinkle the cubed Chicken over the chicken, followed by an even layer of Swiss cheese.
Pour the cream sauce over the cheese layer, spreading it out evenly with a spatula.
Add the Topping
In a small mixing bowl, combine Panko breadcrumbs and melted butter. Stir to coat. Sprinkle the breadcrumbs evenly over the casserole.
Bake the Casserole
Bake in the preheated oven for 25-30 minutes or until the top is golden brown and the casserole is bubbling around the edges.