01 - Set oven temperature to 375°F and allow to preheat while preparing other components.
02 - Bring a large pot of salted water to a boil. Add the quartered potatoes and simmer until fork-tender, about 15 minutes. Drain thoroughly.
03 - In a large mixing bowl, combine ground beef, ground pork, onion, garlic, breadcrumbs, whole milk, egg, ketchup, Worcestershire sauce, dried thyme, salt, and black pepper. Mix gently until just incorporated, avoiding overmixing.
04 - Lightly grease a 10-inch oven-safe skillet. Evenly press meat mixture into the skillet, shaping smoothly. In a small bowl, combine ketchup, brown sugar, and Dijon mustard. Spread half of glaze mixture over meat surface.
05 - Place the skillet uncovered in the oven and bake for 30 minutes.
06 - While baking, mash drained potatoes with unsalted butter, warmed whole milk, Parmesan cheese (if using), salt, and white pepper until smooth and creamy.
07 - Remove skillet from oven. Carefully spread mashed potatoes in an even layer over the meatloaf. Top potatoes with remaining glaze.
08 - Return the skillet to the oven and bake for an additional 20 minutes, until the top is golden brown and a thermometer inserted in the center of the loaf registers 160°F.
09 - Allow the dish to rest for 5 minutes before slicing and serving directly from the skillet.