01 - Preheat oven to 375°F. Grease a 9x13-inch baking dish with butter or non-stick spray.
02 - Place peeled and cubed potatoes in a large pot and cover with salted water. Bring to a boil and cook until fork-tender, about 15 minutes. Drain well, then mash with butter, milk, salt, and pepper until smooth. Set aside.
03 - In a large mixing bowl, combine ground beef, ground pork, onion, garlic, breadcrumbs, egg, milk, Worcestershire sauce, salt, pepper, thyme, and smoked paprika. Mix gently until just combined without overworking the meat.
04 - Evenly press the seasoned meat mixture into the bottom of the prepared baking dish, creating a uniform flat layer.
05 - In a small bowl, whisk together ketchup, brown sugar, and Dijon mustard. Spread the sauce evenly across the surface of the meat layer.
06 - Carefully spoon the mashed potatoes over the sauced meat layer, spreading gently to cover the entire surface in an even layer.
07 - Sprinkle shredded cheddar cheese evenly 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 melted and golden brown.
09 - Remove from the oven and let the casserole rest for 5 to 10 minutes to set. Garnish with chopped fresh parsley if desired. Serve warm.