01 - Preheat oven to 375°F. Grease a 9x13-inch baking dish with butter or nonstick spray.
02 - In a large pot, cook the peeled and cubed potatoes in salted boiling water until fork-tender, approximately 15 minutes. Drain thoroughly, 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 incorporated, being careful not to overwork the mixture.
04 - Transfer the meat mixture into the prepared baking dish and press it evenly across the bottom to form a uniform base layer.
05 - In a small bowl, whisk together the ketchup, brown sugar, and Dijon mustard until smooth. Spread the sauce evenly over the pressed meat layer.
06 - Gently spoon the prepared mashed potatoes over the sauced meat layer, spreading carefully to cover the surface evenly without disturbing the layers beneath.
07 - Sprinkle the shredded cheddar cheese evenly across the top of the mashed potato layer.
08 - Bake uncovered for 40 to 45 minutes, or until the casserole is bubbling at the edges, the meat is cooked through, and the cheese is melted and golden brown on top.
09 - Remove from the oven and let the casserole rest for 5 to 10 minutes to allow the layers to set. Garnish with chopped fresh parsley if desired, then serve warm.