01 - Preheat oven to 375°F. Grease a 9x13-inch baking dish with butter or non-stick spray.
02 - In a large pot, cook the peeled and cubed potatoes in salted boiling water until fork-tender, about 15 minutes. Drain thoroughly, then mash with butter, milk, salt, and pepper until smooth. Set aside.
03 - In a large bowl, combine the ground beef, ground pork, chopped onion, minced garlic, breadcrumbs, egg, milk, Worcestershire sauce, salt, black pepper, thyme, and smoked paprika. Mix gently until just incorporated—do not overmix to keep the texture tender.
04 - Press the meat mixture evenly into the bottom of the prepared baking dish, forming a uniform layer.
05 - In a small bowl, stir together the ketchup, brown sugar, and Dijon mustard. Spread the sauce evenly across the surface of the meat layer.
06 - Spoon the mashed potatoes over the sauced meat layer and spread gently to cover evenly, sealing the edges to prevent the sauce from bubbling through.
07 - Sprinkle the shredded cheddar cheese evenly over 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 melted and golden brown on top.
09 - Remove from the oven and let rest for 5 to 10 minutes. Garnish with chopped fresh parsley if desired and serve warm.