01 - Preheat oven to 375°F. Grease a 9x13-inch baking dish with butter or nonstick cooking spray.
02 - Place peeled and cubed potatoes in a large pot, cover with cold salted water, and bring to a boil. Cook for 15 minutes or until fork-tender. Drain thoroughly, then mash with butter, milk, salt, and pepper until smooth. Set aside.
03 - In a large mixing bowl, combine ground beef, ground pork, chopped onion, minced garlic, breadcrumbs, egg, milk, Worcestershire sauce, salt, black pepper, dried thyme, and smoked paprika. Mix gently until just combined — avoid overmixing to keep the texture tender.
04 - Press the meat mixture evenly into the bottom of the prepared baking dish, forming a uniform base 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 - Spoon the prepared mashed potatoes over the sauced meat layer, spreading gently and evenly to cover completely.
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 golden brown.
09 - Remove from the oven and let rest for 5 to 10 minutes before slicing. Garnish with chopped fresh parsley if desired. Serve warm.