Cook the ground beef and chopped onion in a skillet over medium heat until the beef is fully browned and the onion softens. Drain the excess grease.
Transfer the beef and onion to the slow cooker. Add the pasta sauce, stir, cover, and cook on LOW for 3 hours.
Stir the uncooked egg noodles into the sauce. Cover and cook on HIGH for 20 to 30 minutes, checking at 20 minutes. Add a small splash of water if the sauce is too thick for the noodles to cook evenly.
Sprinkle the shredded mozzarella over the tender noodles. Cover for 5 to 10 minutes until melted, then serve hot.