01 - Preheat oven to 375°F. Lightly grease a 9x13-inch baking dish.
02 - In a saucepan over medium heat, melt unsalted butter. Add diced onion and minced garlic; sauté for 2–3 minutes until soft and translucent.
03 - Whisk in all-purpose flour and cook for 1 minute, stirring constantly to eliminate any raw flour taste.
04 - Gradually pour in whole milk while whisking continuously until no lumps remain. Simmer for 3–4 minutes, stirring until the sauce thickens slightly.
05 - Blend in Dijon mustard, kosher salt, black pepper, and smoked paprika. Remove from heat once the mixture is smooth.
06 - Arrange half the diced chicken in the prepared dish. Top with all ham, then half the shredded Swiss cheese. Repeat with remaining chicken and Swiss cheese.
07 - Evenly pour the creamy sauce over layered ingredients, ensuring complete coverage.
08 - In a bowl, mix Panko breadcrumbs with grated Parmesan. Distribute breadcrumb mixture evenly over the top.
09 - Bake uncovered at 375°F for 30–35 minutes, until topping is golden and contents are bubbling. Let stand 5 minutes before serving.