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 unsalted 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 incorporated—avoid overmixing 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 mixture evenly across the surface of the meat layer.
06 - Spoon the prepared mashed potatoes over the sauced meat layer, spreading gently and evenly to cover the entire surface.
07 - Sprinkle the shredded cheddar cheese evenly over the mashed potato layer.
08 - Bake uncovered for 40–45 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 rest for 5–10 minutes before slicing. Garnish with chopped fresh parsley if desired and serve warm.