Preheat the oven to 375 degrees and lightly grease a loaf pan or baking dish. In a large bowl, combine the ground beef, breadcrumbs, egg, onion, 2 cloves of the minced garlic, Worcestershire sauce, 1/2 teaspoon salt, and 1/4 teaspoon black pepper. Mix until just combined, then shape the mixture into a loaf and place it in the prepared pan.

Spread the ketchup evenly over the top of the meatloaf. Bake for 50 to 55 minutes until the internal temperature reaches 160 degrees.

While the meatloaf bakes, place the potatoes in a large pot and cover with cold water. Add the remaining 1/2 teaspoon salt and bring to a boil. Cook for 15 to 18 minutes until the potatoes are fork-tender.

Drain the potatoes and return them to the pot. Add the butter, milk, remaining 2 cloves of minced garlic, and remaining 1/4 teaspoon black pepper. Mash until smooth and creamy.

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