Preheat your oven to 350°F and spray a 9x13-inch casserole dish with nonstick cooking spray.
Evenly spread the shredded chicken into the bottom of your prepared baking dish.
Layer the sliced black forest chicken ham over the shredded chicken.
Place slices of Swiss cheese evenly over the ham.
In a saucepan over medium heat, melt unsalted butter. Whisk in Dijon mustard, lemon juice if using, garlic powder, salt, nutmeg if using, and cayenne pepper if using.
Sprinkle in all-purpose flour while continuing to whisk until it forms a paste.
Slowly add whole milk to your roux, one quarter cup at a time, whisking after each addition until smooth.
Allow your sauce to simmer for about four minutes until it thickens slightly while whisking occasionally.
Pour your creamy sauce over the cheese layer in the casserole dish, spreading it evenly.
In a small bowl, combine Panko breadcrumbs with softened butter. Mix until crumbly and then spread this topping evenly over your casserole.
Place your casserole in the preheated oven uncovered and bake for about 30–40 minutes or until bubbly and golden brown on top.
Once baked, remove from the oven and let it sit for five minutes before serving.