01 - Preheat oven to 375°F. Grease a 9x13-inch baking dish with butter or nonstick cooking spray.
02 - Place peeled and cubed potatoes in a large pot and cover with cold salted water. Bring to a boil and cook for approximately 15 minutes until fork-tender. Drain thoroughly, then mash with butter, milk, salt, and pepper until smooth. Set aside.
03 - In a large mixing bowl, combine ground beef, ground pork, chopped onion, minced garlic, breadcrumbs, egg, milk, Worcestershire sauce, salt, black pepper, dried thyme, and smoked paprika. Mix gently until just incorporated without overworking the meat.
04 - Press the meat mixture evenly into the bottom of the prepared baking dish, forming a uniform flat layer.
05 - In a small bowl, stir together ketchup, brown sugar, and Dijon mustard until smooth. Spread the sauce evenly across the surface of the meat layer.
06 - Spoon the prepared mashed potatoes over the sauced meat layer and spread gently to cover evenly, sealing the edges.
07 - Sprinkle shredded cheddar cheese uniformly over the mashed potato layer.
08 - Bake uncovered for 40 to 45 minutes until the casserole is bubbling at the edges, the meat is cooked through, and the cheese is golden brown.
09 - Remove from the oven and let rest for 5 to 10 minutes before slicing. Garnish with chopped fresh parsley if desired. Serve warm.