Preheat oven to 180 C. Lightly grease a 9x13 baking dish with butter. Place the shredded chicken on the bottom of the dish, next place the ham slices, the Swiss cheese over the ham.
For the sauce, over medium heat, melt the butter in a large sauce pan. Whisk in the flour until smooth. Continue to whisk mixture while slowly adding milk, whisk until sauce thickens, about 10 minutes. Add lemon juice, brown mustard, paprika, and black pepper. Bring sauce to a low simmer again, whisk continuously. Once it has reached desired thickness, remove from heat and pour sauce evenly over the casserole.
For the topping: In a small sauce pan, melt the butter and stir in panko breadcrumbs, garlic, parsley, and black pepper. Sprinkle topping mixture over the top of the casserole.
Bake for 35 minutes until topping is golden brown. Let res for 5-10 minutes before cutting and serving. Any green veggy on the side would go perfect with this dish.