In a large bowl, combine the ground beef, breadcrumbs, egg, onion, half the minced garlic, salt, and pepper. Mix gently with your hands until just combined.
Shape the mixture into a loaf that will fit inside your slow cooker. Place it in the center of the slow cooker.
Stir together the ketchup, brown sugar, and Worcestershire sauce in a small bowl. Spread this mixture evenly over the top of the meatloaf.
Cover and cook on low for 6 to 7 hours until the meatloaf reaches an internal temperature of 160°F.
About 30 minutes before the meatloaf is done, place the potatoes in a large pot and cover with water. Bring to a boil and cook until the potatoes are fork-tender, about 15 minutes.
Drain the potatoes, then return them to the pot. Add the butter, milk, and remaining minced garlic. Mash until smooth and creamy. Slice the meatloaf and serve it with the mashed potatoes, sprinkled with fresh parsley.