Preheat the oven. Preheat the oven to 375°F (190°C). Lightly grease a 9×9-inch baking dish. An 8×8-inch dish can also be used for thicker layers.
Prepare the beef mixture. Add the ground beef, breadcrumbs, egg, milk, onion, garlic powder, salt, black pepper, Worcestershire sauce, and 1/4 cup ketchup to a large bowl. Gently mix until evenly combined without overworking the beef.
Form the base. Press the meat mixture evenly across the bottom of the prepared baking dish. Spread the remaining 1/4 cup ketchup over the surface.
Bake. Bake for 20–25 minutes, or until the ground beef reaches a safe internal temperature of 160°F (71°C).
Add the toppings. Carefully spread the mashed potatoes over the cooked meatloaf base. Sprinkle evenly with cheddar cheese and chopped cooked turkey ham.
Finish baking. Return the Cheesy Loaded Meatloaf Casserole to the oven and bake for another 10–15 minutes, until the cheese is completely melted and bubbling around the edges.
Rest and serve. Remove the casserole from the oven and garnish with green onions or chives if desired. Rest for 5–10 minutes before slicing and serving.