01 - Preheat oven to 375°F. Grease a 9x13-inch baking dish with butter or nonstick spray.
02 - Place peeled and cubed potatoes in a large pot, cover with salted water, and bring to a boil. Cook for about 15 minutes until fork-tender. Drain well, then 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 incorporated, being careful not to overwork the mixture.
04 - Evenly press the seasoned 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. Spread the sauce evenly across the surface of the meat layer.
06 - Gently spoon the mashed potatoes over the sauced meat layer, spreading carefully to cover evenly without disturbing the layers beneath.
07 - Sprinkle shredded cheddar cheese evenly over the mashed potato layer.
08 - Bake uncovered for 40 to 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 oven and let rest for 5 to 10 minutes to allow the layers to set. Garnish with chopped fresh parsley if desired, then serve warm.