In a large mixing bowl, combine the ground beef, breadcrumbs, milk, finely chopped onion, eggs, minced garlic, ¼ cup ketchup, Worcestershire sauce, salt, black pepper, and mustard. Mix until just combined, being careful not to overmix.
Shape the meat mixture into a loaf shape that will fit inside your crockpot.
In a small bowl, mix together the brown sugar and ¼ cup ketchup to form the glaze.
Place the meatloaf into the crockpot and pour the glaze evenly over the top.
Cover the crockpot and set it on low. Cook for 6 hours or until the internal temperature reaches 160°F (70°C).
Once cooked, carefully remove the meatloaf from the crockpot using two spatulas and let it rest for about 10 minutes before slicing.