01 - Preheat oven to 375 degrees Fahrenheit. Lightly grease a 9x13-inch baking dish to prevent sticking.
02 - Melt unsalted butter in a saucepan over medium heat. Add finely diced onion and minced garlic. Sauté for 2 to 3 minutes until soft and fragrant.
03 - Whisk all-purpose flour into the butter mixture. Cook for 1 minute, stirring constantly. Gradually add whole milk while whisking, ensuring a smooth consistency. Simmer for 3 to 4 minutes until sauce thickens slightly.
04 - Stir Dijon mustard, kosher salt, ground black pepper, and smoked paprika into the sauce. Remove from heat.
05 - In the prepared baking dish, arrange half the chicken breast, then spread all the diced ham, followed by half of the shredded Swiss cheese. Repeat layers with the remaining chicken and Swiss cheese.
06 - Pour prepared sauce evenly over the layered ingredients. In a separate bowl, mix Panko breadcrumbs with grated Parmesan cheese, then distribute mixture evenly over the top.
07 - Bake uncovered for 30 to 35 minutes, or until topping is golden and the casserole is bubbling at the edges.
08 - Remove from oven and let rest for 5 minutes before slicing and serving.