Trim fat from beef, and cut into 1-inch cubes. Place beef in an electric slow cooker; stir in onion and barbecue sauce. Cover with lid; cook on low-heat setting for 8 hours.
Remove beef and onions from slow cooker with a slotted spoon. Shred beef mixture with 2 forks; set aside. Combine cornstarch and water, and add cornstarch mixture to barbecue sauce mixture in slow cooker, stirring well. Cover with lid, and cook on high-heat setting for 1 minute. Return the shredded-beef mixture to slow cooker; cover and cook 10 minutes. Serve on rolls.
Note: You can freeze the barbecued-beef mixture in an airtight container for up to 3 months.