01 - Preheat oven to 375°F. Grease a 9x13-inch baking dish with butter or non-stick spray.
02 - In a large pot, cook peeled and cubed potatoes in salted boiling water until fork-tender, about 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, 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 layer.
05 - In a small bowl, whisk together ketchup, brown sugar, and Dijon mustard. Spread the sauce evenly over the meat layer.
06 - Gently spoon the prepared mashed potatoes over the sauced meat layer, spreading evenly to the edges with a spatula.
07 - Sprinkle shredded cheddar cheese evenly across the top of the mashed potato layer.
08 - Bake uncovered for 40–45 minutes until the casserole is bubbling, the meat is cooked through, and the cheese is golden brown.
09 - Remove from the oven and let rest for 5–10 minutes before slicing. Garnish with chopped fresh parsley if desired and serve warm.