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 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, onion, garlic, breadcrumbs, egg, milk, Worcestershire sauce, salt, pepper, thyme, and paprika. Mix gently until just combined without overworking the meat.
04 - Evenly press the meat mixture into the bottom of the prepared baking dish, forming a uniform layer.
05 - In a small bowl, whisk together the ketchup, brown sugar, and Dijon mustard. Spread the sauce evenly over the pressed meat layer.
06 - Gently spoon the mashed potatoes over the sauced meat layer and spread into an even coating, sealing the edges to prevent the sauce from bubbling over.
07 - Sprinkle the shredded cheddar cheese evenly across the top of 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 golden brown.
09 - Remove from the oven and let rest for 5 to 10 minutes. Garnish with chopped fresh parsley if desired. Serve warm.