Preheat oven to 350°F (about 175°C). Spray a 9×13 inch casserole dish with cooking spray.

Layer the shredded rotisserie chicken evenly on the bottom of the dish. Then layer the cubed ham evenly on top of the chicken.

For the cheese sauce: In a saucepan over medium heat, melt the 4 tablespoons butter. Sprinkle in the ¼ cup flour and cook, stirring, for 1‑2 minutes until the mixture turns dry and clumpy.

Gradually whisk in the 2 cups milk until smooth. Add the Dijon mustard, lemon juice, garlic powder, and salt. Simmer for 3‑5 minutes, whisking constantly, until the sauce thickens.

Remove from heat and stir in the Swiss cheese and Parmesan cheese. Taste and add more salt if needed.

Pour the cheese sauce evenly over the chicken and ham layers.

For the panko topping: In a small bowl, combine the Panko bread crumbs with the melted butter. Mix well and sprinkle over the top of the casserole.

Bake the casserole uncovered for about 30‑35 minutes or until the breadcrumb topping is golden brown and the casserole is bubbly.

Let it cool for about 5‑10 minutes before serving. You can serve it as is, or place it over cooked pasta or rice if you want to stretch the meal.