In large skillet, cook ground beef, onions, celery and bell pepper over medium-high heat for 5 to 7 minutes or until beef is thoroughly cooked, stirring frequently. Drain.
In 3 1/2 to 4-quart slow cooker, combine ground beef mixture and all remaining ingredients except buns; mix well.
Cover; cook on low setting for 4 to 6 hours.
Stir; spoon beef mixture into buns.