Step 1: Preheat the Oven

1. Preheat your oven to 375°F (190°C). Grease a 9×13-inch baking dish and set it aside.

Step 2: Prepare the Chicken Layer

1. Spread the cooked chicken evenly across the bottom of the prepared baking dish.

Step 3: Add the Ham and Cheese

1. Scatter the chopped ham evenly over the chicken layer.
2. Lay the slices of Swiss cheese over the ham, or sprinkle shredded Swiss cheese evenly across the top.

Step 4: Make the Creamy Sauce

1. In a medium saucepan, melt the butter over medium heat. Once melted, whisk in the flour to form a smooth paste (roux). Cook for 1–2 minutes, stirring constantly.
2. Gradually whisk in the milk, ensuring no lumps form. Add the garlic powder, onion powder, Dijon mustard, paprika, salt, and pepper.
3. Continue to cook, whisking frequently, until the sauce thickens and coats the back of a spoon (about 3–5 minutes).
4. Stir in the grated Parmesan cheese and mix until melted and smooth.

Step 5: Assemble the Casserole

1. Pour the creamy sauce evenly over the chicken, ham, and cheese layers in the baking dish.

Step 6: Prepare the Breadcrumb Topping

1. In a small bowl, mix the panko breadcrumbs, melted butter, grated Parmesan, and dried parsley. Toss until the breadcrumbs are evenly coated.
2. Sprinkle the breadcrumb mixture evenly over the casserole.

Step 7: Bake

1. Bake the casserole in the preheated oven for 30–35 minutes, or until the topping is golden brown and the sauce is bubbling around the edges.

Step 8: Cool and Serve

1. Remove the casserole from the oven and let it rest for 5–10 minutes before serving. This allows the layers to set and makes it easier to portion.