Place beef in a 5-quart slow cooker. Add the celery, onion and green pepper. In a bowl, combine the ketchup, tomato paste, brown sugar, vinegar, chili powder, lemon juice, molasses, salt, Worcestershire sauce and mustard. Pour over beef mixture. Cover and cook on low for 8-9 hours or until meat is tender.
Skim fat from cooking juices if necessary. Shred beef. Toast rolls if desired. Use a slotted spoon to serve beef on rolls.
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