Preheat the oven to 350 degrees F. In a large bowl, combine the ground beef, breadcrumbs, egg, onion, 1/4 cup milk, 1 teaspoon salt, and black pepper. Mix just until the ingredients hold together.

Shape the mixture into a loaf and place it in a loaf pan. Spread the ketchup evenly over the top.

Bake the meatloaf for 55 to 60 minutes until it reaches an internal temperature of 160 degrees F.

While the meatloaf bakes, place the cubed potatoes in a large pot and cover with water. Bring to a boil over medium-high heat and cook for 15 to 20 minutes until the potatoes are fork tender.

Drain the potatoes and return them to the pot. Add the butter, 1/2 cup milk, and 1/2 teaspoon salt. Mash until smooth and creamy.

Remove the meatloaf from the oven and let it rest for 5 minutes before slicing. Serve slices alongside the mashed potatoes.