01 - Preheat oven to 375°F (190°C). Coat a 9x13-inch baking dish with nonstick spray or butter.
02 - Melt unsalted butter in a saucepan over medium heat. Add diced onion and minced garlic, cooking for 2 to 3 minutes until softened and fragrant.
03 - Whisk in all-purpose flour and cook, stirring continually, for 1 minute. Slowly add whole milk while whisking to prevent lumps, then simmer for 3 to 4 minutes until the sauce begins to thicken.
04 - Incorporate Dijon mustard, kosher salt, black pepper, and smoked paprika into the sauce. Remove saucepan from heat.
05 - In the prepared baking dish, distribute half of the cooked chicken, all diced ham, and half the Swiss cheese. Repeat layering with remaining chicken and Swiss cheese.
06 - Evenly pour the prepared sauce over the layered ingredients in the baking dish.
07 - Combine Panko breadcrumbs with grated Parmesan cheese in a mixing bowl. Sprinkle the mixture evenly across the casserole.
08 - Bake uncovered for 30 to 35 minutes until the topping is golden and the casserole is bubbling at the edges.
09 - Allow casserole to rest for 5 minutes before serving to preserve structure and enhance flavor.