Preheat the oven: Preheat the oven to 375°F (190°C). Lightly grease a 9×9-inch baking dish, or use an 8×8-inch dish for a thicker casserole.
Prepare the meatloaf mixture: Add the ground beef, breadcrumbs, egg, milk, chopped onion, garlic powder, salt, black pepper, halal-certified Worcestershire sauce, and half of the ketchup to a large bowl. Mix gently until the ingredients are evenly combined.
Bake the meatloaf layer: Press the mixture evenly into the prepared baking dish. Spread the remaining ketchup over the surface. Bake for 20–25 minutes, or until the beef is fully cooked and reaches a safe internal temperature.
Add the potatoes and cheese: Carefully spread the mashed potatoes over the cooked meatloaf layer. Sprinkle the shredded cheddar evenly over the potatoes, followed by the crumbled turkey ham.
Finish baking: Return the dish to the oven and bake for another 10–15 minutes, until the cheese is completely melted and bubbling around the edges.
Garnish and serve: Remove the casserole from the oven and sprinkle with green onions or chives, if desired. Let it rest for a few minutes before slicing and serving.