Trim fat from the brisket.
If necessary, cut the brisket to fit into the crock pot.
Place onions, bay leaf, and brisket.
In a separate bowl, combine beer, chili sauce, sugar, thyme, salt, pepper and garlic.
Pour over the brisket once mixed.
Cover, and cook on low heat for 10 to 12 hours or on high heat for 5 to 6 hours.
Transfer brisket and onions to a platter and keep warm.
Discard the bay leaf.
For gravy, skim fat from cooking juices.
Measure 2 ½ cups juices and place in a saucepan.
Combine cornstarch and water and add to the saucepan.
Cook and stir until thick and bubbly, then cook and stir for 2 minutes more.