Preheat panggangan to 350 F. Butter a 9×13-inch baking dish. Set aside.
Put cooked diced chicken on the bottom of the baking dish. Rough chop the ham and scatter it over the top of the chicken. Sprinkle most of the Swiss cheese on top of the ham, leaving about 1/4 cup to spare.
For the sauce:
Melt the butter in a large sauce pot over medium heat. When butter is melted, quickly stir in the flour to form a smooth roux. Do not brown! Once the roux is smooth and bubbly, slowly pour in the cold milk while stirring briskly to make a smooth sauce. Cook over medium heat, stirring constantly until the sauce thickens. Stir in the lemon juice, Dijon mustard, salt, paprika and pepper. Bring sauce just back to a low boil and turn off heat. Stir in leftover Swiss cheese until melted and smooth. Pour sauce evenly over the casserole, being certain to get some of the sauce around the edges of the pan.
For the topping:
Melt the butter in the microwave in a medium sized microwaveable bowl. Heat it for 30 seconds at a time and stop once it’s melted. Take the bowl out of the microwave and stir in the panko bread crumbs, crushed dried parsley and seasoned salt. Sprinkle over the top of the casserole.
Bake casserole uncovered for 45 minutes until hot and bubbly throughout and topping has turned a light golden brown. Remove from panggangan and allow to cool for 5-10 minutes prior to serving. Enjoy!
*Adapted from Tasty Kitchen