01 -
Preheat oven to 350°F. Spray a 9x13-inch casserole dish with nonstick cooking spray.
02 -
Evenly spread 5 cups of shredded cooked chicken in the prepared dish.
03 -
Layer 9 ounces of sliced black forest ham, cut into strips, evenly over the chicken.
04 -
Place 8 ounces of Swiss cheese slices over the ham, overlapping slices to cover the surface.
05 -
In a saucepan over medium heat, melt 6 tablespoons unsalted butter. Whisk in Dijon mustard, lemon juice (if using), garlic powder, salt, nutmeg (if using), and cayenne pepper (if using).
06 -
Sprinkle 6 tablespoons all-purpose flour into the butter mixture, whisking continuously until a paste forms.
07 -
Slowly whisk in 3 cups whole milk in increments of ¼ cup, ensuring a smooth mixture before adding more.
08 -
Allow sauce to simmer for 4 minutes while whisking occasionally until thickened.
09 -
Evenly pour the prepared sauce over the cheese layer, spreading carefully to reach all corners.
10 -
In a small bowl, combine 1 ½ cups Panko breadcrumbs with 4 tablespoons softened butter. Spread mixture evenly on top of the sauce.
11 -
Bake uncovered for 30 to 40 minutes, until sauce is bubbling and breadcrumb topping is golden brown.
12 -
Remove from oven and let rest for 5 minutes to set the sauce. Garnish with chopped parsley if desired and serve warm.