Ingredients :

5 cups shredded chicken
9 ounces sliced deli-style black forest ham, cut into strips (about 9 slices – see notes below)
8 ounces sliced Swiss cheese (about 12 thin slices)
Sauce:
6 Tablespoons unsalted butter
1 Tablespoon Dijon mustard
1 Tablespoon freshly squeezed lemon juice (optional)
1 teaspoon garlic powder
½ teaspoon salt
¼ teaspoon nutmeg (optional)
¼ teaspoon cayenne pepper (optional)
6 Tablespoons all-purpose flour
3 cups whole milk
Topping:
1 ½ cups Panko breadcrumbs
4 Tablespoons unsalted butter, softened to room temperature (see notes below)
chopped parsley (optional)

Instructions :

Preheat the oven to 350°F and spray a 9×13-inch casserole dish with nonstick cooking spray.
Evenly spread 5 cups shredded chicken into the baking dish.
Next, evenly layer 9 ounces sliced deli-style black forest ham, cut into strips over the chicken.
Then place 8 ounces sliced Swiss cheese evenly over the ham, overlapping the cheese slices to fit into the casserole dish.
Melt 6 Tablespoons unsalted butter in a saucepan over medium heat. Whisk in t1 Tablespoon Dijon mustard, 1 Tablespoon freshly squeezed lemon juice (if using), 1 teaspoon garlic powder, 1/2 teaspoon salt, 1/4 teaspoon nutmeg (if using) and 1/4 teaspoon cayenne pepper (if using).
Sprinkle in 6 Tablespoons all-purpose flour and continue whisking over medium heat until it forms a paste.
Slowly add 3 cups whole milk, a quarter of a cup at a time, whisking after each addition until the mixture is smooth, before adding the next bit of milk.
Once all the milk has been added, allow it to simmer for 4 minutes or until the sauce has thickened (whisking occasionally.)
Pour the sauce over the cheese layer and spread it evenly around as best as possible, making sure to get in all the corners.
Combine the 1 ½ cups Panko breadcrumbs and 4 Tablespoons unsalted butter, softened to room temperature butter in a small bowl and spread this mixture evenly over the casserole dish as your final layer.
Bake in the oven uncovered for 30-40 minutes, or until the sauce is bubbling and the Panko topping is golden brown.
Remove the casserole dish from the oven and let it sit for 5 minutes to allow the sauce to set. Sprinkle with chopped parsley (optional) and serve.