Preheat your oven to 375°F (190°C). Prepare your ingredients for mixing.

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

Flatten the beef mixture into a rectangle, roughly ½-inch thick on a piece of wax paper.

Spread the prepared mashed potatoes evenly down the center of the beef rectangle, leaving a small border around the edges.

Fold the edges of the beef mixture over the mashed potatoes, forming a log shape. Seal tightly.

In a skillet, heat olive oil and sear the meatloaf seam-side down for 5–7 minutes until browned on all sides.

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

Let the meatloaf rest for 10 minutes before slicing and serving.