Preheat the oven to 350°F and spray a 9×13-inch casserole dish with nonstick cooking spray.

Evenly spread 5 cups of shredded chicken into the baking dish.

Layer 9 ounces of sliced deli-style black forest ham over the chicken.

Place 8 ounces of sliced Swiss cheese evenly over the ham, overlapping the slices to fit into the dish.

In a saucepan, melt 6 tablespoons of unsalted butter over medium heat.

Whisk in 1 tablespoon of Dijon mustard, 1 tablespoon of lemon juice (if using), 1 teaspoon of garlic powder, ½ teaspoon of salt, ¼ teaspoon of nutmeg (if using), and ¼ teaspoon of cayenne pepper (if using).

Sprinkle in 6 tablespoons of all-purpose flour and continue whisking until it forms a paste.

Gradually add 3 cups of whole milk, ¼ cup at a time, whisking thoroughly after each addition until smooth.

Allow the mixture to simmer for 4 minutes, whisking occasionally until it thickens.

Pour the sauce over the cheese layer, spreading it evenly.

In a small bowl, combine 1 ½ cups of Panko breadcrumbs and 4 tablespoons of softened butter.

Spread this mixture evenly over the casserole as the final layer.

Bake uncovered for 30-40 minutes until the sauce is bubbling and the topping is golden brown.

Remove the dish from the oven and let it sit for 5 minutes to set.

Sprinkle with chopped parsley (optional) and serve.