Combine ground beef, onion, bread crumbs, broth, egg, Parmesan cheese, 2 tablespoons ketchup, Worcestershire sauce, Italian seasoning, parsley, and garlic powder in a bowl using your hands. Form into a loaf. Center the loaf in the bottom of a slow cooker, leaving room on the sides.
Coat meatloaf with 1/4 cup ketchup and surround with potatoes and carrots. Cook on High, about 2 hours. Reduce heat setting to Low; cook until meatloaf is no longer pink in the center and vegetables are tender, 4 to 6 hours more. An instant-read thermometer inserted into the center should read at least 160 degrees F (70 degrees C).