Preheat oven to 325 derees F. Slice onions and arrange in center of a roasting pan that will hold the brisket snugly. Place brisket on top, fat side up.
Whisk together the remaining ingredients; pour over brisket. Cover pan with heavy aluminum foil, sealing it tightly. Bake 3-1/2 hours or until tender.
Remove from oven; transfer meat to a cutting board and cover with foil.
Transfer the onions to a blender with a ladle or two of the liquid from the pan. Blend until smooth; stir back into remaining liquid in pan.
Slice brisket across the grain and then, if desired, cut the slices in half. Place slices in gravy in the pan. Store leftovers in a covered container in the refrigerator up to 3 days, or freeze up to 2 months. Reheat meat in the gravy in a 350 degrees F oven in a pan covered with foil. Or microwave, covered, 30 seconds at a time until hot. Makes 6 servings plus leftovers.