Brown the ground beef and diced onion in a skillet over medium heat until the beef is no longer pink. Drain any excess fat.
Transfer the beef and onion mixture to the slow cooker. Add the cubed potatoes, beef broth, ketchup, yellow mustard, garlic powder, salt, and pepper.
Stir everything together, cover, and cook on low for 6 to 7 hours until the potatoes are fork-tender.
Stir in the milk and shredded cheddar cheese until the cheese melts fully into the soup.
Taste and adjust salt and pepper if needed before serving.