1In large nonstick skillet, cook ground beef and onion over medium heat for 5 minutes or until beef is thoroughly cooked, stirring occasionally. Drain.
2Place beef mixture in 3 1/2- to 4-quart slow cooker. Add all remaining ingredients except sour cream; stir to combine.
3Cover; cook on low setting for at least 5 hours. Serve with sour cream if desired.