Preheat the oven to 375 degrees. In a large bowl, combine the ground beef, breadcrumbs, egg, onion, garlic, 1/4 cup milk, ketchup, 1 teaspoon salt, and 1/2 teaspoon black pepper. Mix until evenly blended.
Shape the mixture into an oval loaf on a baking sheet. Use a spoon or your fingers to press shallow grooves across the top to create a brain-like pattern.
Bake for 45 minutes until the internal temperature reaches 160 degrees. Let the loaf rest for 5 minutes before slicing.
While the meatloaf bakes, place the cubed potatoes in a pot and cover with water. Boil until tender, about 15 minutes.
Drain the potatoes. Add the butter, 1/2 cup milk, 1/2 teaspoon salt, and 1/4 teaspoon black pepper. Mash until smooth and creamy.