Preheat the oven to 375°F (190°C). Prepare a loaf pan by lightly greasing it or lining it with a sheet of parchment paper for easier removal.
In a large mixing bowl, incorporate the ground chicken with the breadcrumbs, beaten eggs, milk, finely diced onion, garlic, Dijon mustard, salt, pepper, paprika, and parsley. Mix thoroughly until just combined, ensuring not to overmix to retain tenderness.
Press half of the seasoned chicken mixture into the bottom of the prepared loaf pan, smoothing it into an even layer.
Lay the slices of ham evenly across the surface of the chicken base, then place the cheese slices directly over the ham to create the central filling layer.
Carefully spread the remaining chicken mixture over the top, sealing around the edges to encase the filling completely and prevent cheese from escaping during baking.
Transfer the loaf to the oven and bake for 45 to 50 minutes, or until the internal temperature reaches 165°F (74°C) and the meat is fully cooked through.
If using the optional topping, mix the Dijon mustard and mayonnaise together, then spread this mixture over the surface of the loaf in the final 5 minutes of baking. Sprinkle the shredded Swiss cheese over the top and return to the oven until the cheese has melted and turned golden.
Allow the finished loaf to rest for 10 minutes before slicing. Serve hot, accompanied by sides such as roasted vegetables, mashed potatoes, or a crisp garden salad.