Preheat oven to 350° F.
Place chopped onions in bottom of roasting pan.
Rub brisket with garlic.
Place brisket in roasting pan on top of onions.
Place remaining ingredients in roasting pan.
Place in oven and cook, covered, for 45 minutes at 350° F.
After 45 minutes, reduce heat to 275° F.
Continue to cook until tender, for up to 6 hours.
Remove from oven, cool ½ hour, and then slice across the grain.