Sauté the onion in butter on stovetop. When soft and fragrant, put into CrockPot.
Add remaining ingredients to CrockPot and place lid on top of the slow cooker.
If meatballs are frozen, cook on HIGH for 4-6 hours. If meatballs are thawed, cook on LOW 4-5 hours. You may cook on LOW all day, if desired, just use frozen meatballs.
EDIT: I forgot to list, but I threw in some cooked macaroni noodles the last 20 minutes, or so, of cooking :)
Great recipe to throw in the CrockPot on a busy workday and have a complete meat/veggie meal waiting when you return. This recipe uses canned vegetables for convenience, but feel free to use fresh veggies as time allows for your preparation.