01 - Preheat the 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 for approximately 15 minutes until fork-tender. Drain thoroughly, then mash together with butter, milk, salt, and pepper until smooth. Set aside.
03 - In a large mixing bowl, combine ground beef, ground pork, finely chopped onion, minced garlic, breadcrumbs, egg, milk, Worcestershire sauce, salt, black pepper, dried thyme, and smoked paprika. Gently fold until evenly incorporated, being careful not to overwork the mixture.
04 - Press the seasoned meat mixture evenly 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 prepared mashed potatoes over the sauced meat layer, spreading gently to cover completely without disturbing the sauce beneath.
07 - Sprinkle shredded cheddar cheese evenly over the mashed potato surface, ensuring full coverage for a golden, bubbly finish.
08 - Bake uncovered on the center rack 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 allow the layers to set. Garnish with chopped fresh parsley if desired and serve warm.