Preheat your oven to 375°F (190°C).

In a large mixing bowl, combine ground beef, finely chopped onion, breadcrumbs, egg, milk, Worcestershire sauce, salt, and pepper. Mix gently until just combined.

Transfer the meat mixture onto wax paper and flatten it into a rectangle about 1/2 inch thick.

Spread a layer of prepared mashed potatoes on the center of the flattened meat mixture.

Fold the edges of the meat mixture over the mashed potatoes, sealing the edges well.

Heat olive oil in a large skillet over medium heat. Sear the meatloaf for 5–7 minutes, turning until all sides are browned.

Transfer the seared meatloaf to a baking dish and brush the top with ketchup. Bake for 35–40 minutes or until the internal temperature reaches 160°F (71°C).

Allow the meatloaf to rest for about 10 minutes before slicing.