01 - Preheat oven to 375°F. Grease a 9x13-inch baking dish and set aside.
02 - In a large pot, cook peeled and cubed potatoes in salted boiling water until tender, approximately 15 minutes. Drain thoroughly and mash 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. Mix gently until just combined, being careful not to overwork 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 ketchup, brown sugar, and Dijon mustard until smooth. Spread the sauce evenly over the meat layer.
06 - Gently spoon the mashed potatoes over the sauced meat layer and spread into an even coverage, sealing the edges.
07 - Sprinkle shredded cheddar cheese evenly across the entire surface 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.
09 - Remove from oven and let rest for 5 to 10 minutes before slicing. Garnish with chopped fresh parsley if desired and serve warm.