Place half of onion rings in bottom of a 4-quart electric slow cooker. Trim fat from meat, and cut into large pieces to fit in slow cooker; sprinkle with pepper and salt. Dredge meat in flour; place on top of onion, sprinkling with any remaining flour. Add remaining half of onion.
Combine chili sauce and next 4 ingredients in a medium bowl, stirring well. Pour over meat mixture. Cover and cook on high 4 to 5 hours or until meat is tender. Remove brisket, and replace slow cooker cover.
Shred beef, using 2 forks; return meat to hot liquid in slow cooker, stirring well. Spoon meat mixture over toasted roll halves.