01 - Preheat oven to 375°F. Grease a 9x13-inch baking dish and set aside.
02 - In a large pot, cook the peeled and cubed potatoes in salted boiling water until tender, about 15 minutes. Drain well, then mash together with butter, milk, salt, and pepper until smooth. Set aside.
03 - In a large mixing bowl, combine the ground beef, ground pork, finely chopped onion, minced garlic, breadcrumbs, egg, milk, Worcestershire sauce, salt, black pepper, dried thyme, and smoked paprika. Mix gently until just combined without overworking the meat.
04 - Evenly press the meat mixture into the bottom of the prepared baking dish, creating a uniform flat layer.
05 - In a small bowl, whisk together the ketchup, brown sugar, and Dijon mustard. Spread the sauce evenly over the pressed meat layer.
06 - Gently spoon the prepared mashed potatoes over the sauced meat layer, spreading evenly to the edges to fully cover the surface.
07 - Sprinkle the shredded cheddar cheese evenly across the top of the mashed potato layer.
08 - Bake uncovered for 40 to 45 minutes until the casserole is bubbling, the meat is cooked through, and the cheese is golden brown on top.
09 - Remove from the oven and let the casserole rest for 5 to 10 minutes before slicing. Garnish with chopped fresh parsley if desired and serve warm.