Trim fat from meat. If necessary, cut brisket to fit into a 3½- to 6-quart slow cooker. Place the onions, bay leaf, and brisket in the cooker. In a medium bowl combine beer, chili sauce, sugar substitute, garlic, thyme, salt, and pepper; pour over meat.
Cover and cook on low-heat setting for 10 to 12 hours or on high-heat setting for 5 to 6 hours.
Using a slotted spoon, transfer brisket and onions to a serving platter; cover with foil to keep warm. Discard bay leaf.
For gravy, pour juices into a large measuring cup; skim fat. Measure 2½ cups liquid; discard remaining liquid. In a medium saucepan stir together cornstarch and water; stir in the cooking liquid. Cook and stir until thickened and bubbly; cook and stir for 2 minutes more. Serve meat on bread rolls topped with gravy.
Tips: For easy cleanup, line your slow cooker with a disposable slow cooker liner. Add ingredients as directed in recipe. Once your dish is finished cooking, spoon the food out of your slow cooker and simply dispose of the liner. Do not lift or transport the disposable liner with food inside.