Step 1: Preheat the Oven

1. Preheat your oven to 375°F (190°C). Grease a 9×13-inch casserole dish with cooking spray or butter and set aside.

Step 2: Prepare the Creamy Sauce

1. Melt the Butter: In a medium saucepan, melt 3 tbsp butter over medium heat.
2. Make the Roux: Add the flour and whisk constantly for 1-2 minutes until the mixture is smooth and slightly golden.
3. Add the Milk and Cream: Gradually whisk in the milk and heavy cream, stirring to prevent lumps. Cook for 3-4 minutes, or until the sauce begins to thicken.
4. Season the Sauce: Stir in Dijon mustard, garlic powder, onion powder, smoked paprika (if using), salt, and pepper. Taste and adjust seasonings as needed. Remove from heat.

Step 3: Assemble the Casserole

1. Layer the Ingredients: Spread the shredded chicken evenly in the prepared casserole dish. Top with the diced ham, then sprinkle the shredded Swiss and mozzarella cheese over the top.
2. Add the Sauce: Pour the creamy sauce evenly over the chicken, ham, and cheese layers, ensuring everything is well coated.

Step 4: Make the Topping

1. Combine Ingredients: In a small bowl, mix the panko breadcrumbs, melted butter, Parmesan cheese, and dried parsley.
2. Sprinkle the Topping: Evenly spread the breadcrumb mixture over the casserole.

Step 5: Bake

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

Step 6: Rest and Serve

1. Remove the casserole from the oven and let it rest for 5-10 minutes before serving. Garnish with fresh parsley if desired.