Preheat your oven to 350°F (175°C). Grease a loaf pan generously with butter or nonstick spray.

In a medium bowl, combine the ground chicken, egg, finely diced onion, minced garlic, Dijon mustard, grated Parmesan, chopped parsley, salt, pepper, and 1/4 cup of panko breadcrumbs. Mix until just combined.

Lay a piece of parchment paper on your work surface. Pack the meat mixture on the paper, flattening it into about an 8x12-inch rectangle.

Distribute the slices of Black Forest ham evenly over the flattened meat mixture, followed by layers of Swiss cheese.

Using the parchment paper as a guide, roll the meat mixture tightly into a log shape, seam side down in the greased loaf pan. Remove the parchment paper carefully.

In a small bowl, combine remaining panko breadcrumbs with melted butter. Sprinkle this mixture over the top to create a crunchy crust.

Bake for about 75 minutes, until the internal temperature reaches 165°F (75°C). Check for doneness around the one-hour mark.

While the meatloaf is resting, melt a bit of butter in a saucepan, add flour, whisk until smooth, gradually pour in milk, cooking until thickened. Stir in more Dijon mustard and Parmesan until blended, then set aside.

Once rested, slice the meatloaf beautifully and serve warm, drizzling the creamy mustard sauce over each slice.