Heat the oven to 375°F. Lightly grease a 12-cup muffin tin.
In a large bowl, mix the ground beef, egg, breadcrumbs, 1/4 cup milk, diced onion, Worcestershire sauce, salt, and black pepper. Stir just until combined so the meatloaf stays tender.
Divide the beef mixture among 8 muffin cups, pressing it in gently. Spread the reserved 1/4 cup ketchup over the tops.
Bake for 20 to 25 minutes, until the centers reach 160°F. Let the muffins rest in the tin for 5 minutes, then carefully lift them out.
While the meatloaf bakes, place the potato chunks in a saucepan and cover with water. Bring to a boil, then simmer until the potatoes are fork-tender, about 15 to 20 minutes. Drain well.
Mash the potatoes with the butter and warm milk until smooth. Spoon or pipe the mashed potatoes over the rested meatloaf muffins and serve warm.