Heat the oven to 375°F. Lightly grease eight cups of a standard muffin tin.
In a large bowl, combine the ground beef, egg, breadcrumbs, first ¼ cup of milk, diced onion, 1 tablespoon of ketchup, Worcestershire sauce, salt, and black pepper. Mix gently until combined, taking care not to compact the beef.
Divide the mixture among the eight muffin cups. Press it in lightly, then spread the remaining tablespoon of ketchup over the tops.
Bake for 20 to 25 minutes, until the meatloaf muffins are cooked through. Let them rest in the tin for a few minutes, then carefully lift them out.
While the meatloaves bake, place the potato chunks and garlic cloves in a saucepan. Cover with water, bring to a boil, and cook until the potatoes are fork-tender, about 15 to 20 minutes. Drain well.
Mash the potatoes and garlic with the butter, the second ¼ cup of milk, and the remaining ¼ teaspoon of salt. Spoon or pipe the mashed potatoes onto the cooked meatloaf muffins and serve warm.