Trim fat from meat. If necessary, cut brisket to fit into a 3-1/2- to 6-quart slow cooker. In the cooker place onions, bay leaf, and brisket.
In a medium bowl combine beer, chili sauce, sugar substitute, thyme, salt, pepper, and garlic; pour over meat in cooker.
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 meat and onions to a platter; keep warm.
Discard bay leaf.