Preheat oven to 375°F (190°C) and grease a 9x13-inch baking dish.
In a large bowl, combine ground beef, onion, garlic, breadcrumbs, milk, egg, Worcestershire sauce, salt, and black pepper. Mix gently and press evenly into the dish, then spread ketchup over the top. Bake for 25 minutes.
Meanwhile, boil sliced potatoes in salted water until fork-tender, about 15 minutes. Drain and mash with sour cream, milk, and butter until smooth, seasoning with salt and pepper.
Spread mashed potatoes over the meatloaf, then top with cheddar and green onions. Bake for 20 minutes until cheese is melted and bubbly.
Let rest 10 minutes before slicing and serving.