Preheat Oven and Prepare Pan:
Preheat your oven to 350°F (175°C). Grease a loaf pan with butter or non-stick spray. Set aside.

Prepare Meatloaf Mixture:
In a medium bowl, combine the ground chicken, beaten egg, diced onion, minced garlic, Dijon mustard, grated parmesan cheese, chopped parsley, salt, pepper, and 1/4 cup of the panko breadcrumbs. Mix until well combined.

Assemble Meatloaf:
On a piece of parchment paper, press the chicken mixture into a rectangle about 10×12 inches. Lay the slices of ham over the chicken mixture, then place the slices of Swiss cheese on top of the ham.

Roll Meatloaf:
Carefully roll the meatloaf from one short end to the other, using the parchment paper to help. Seal the edges and place the rolled meatloaf seam-side down in the prepared loaf pan.

Add Topping:
Brush the top of the meatloaf with the melted butter and sprinkle with the remaining 3/4 cup of panko breadcrumbs.

Bake:
Bake in the preheated oven for 1 hour, or until the internal temperature reaches 165°F (74°C).

Prepare Sauce:
While the meatloaf is baking, melt the butter for the sauce in a saucepan over medium heat. Whisk in the flour and cook for 1 minute. Gradually add the milk, whisking constantly. Stir in the Dijon mustard and grated parmesan cheese. Cook until the sauce thickens, about 5 minutes.

Serve:
Once the meatloaf is done, let it rest for 10 minutes before slicing. Serve with the creamy Dijon sauce drizzled over the top.