Preheat the oven to 350 degrees.
Scatter the onions in a pan just large enough to fit the meat. Place the brisket on top of the onions. Combine all remaining ingredients in a bowl, and pour the mixture evenly over the meat. Cover tightly with foil, and bake 2 hours. Turn the brisket over, and bake uncovered 1 more hour. Transfer the brisket to a cutting board and wait about 10 minutes before slicing.
Meanwhile strain the cooking liquids into a small sauce pan, pressing hard on the solids (and discarding them), and reduce on a high flame to about 2 ½ cups. Let the brisket cool slightly. Slice thin against the grain. In places where the brisket is very long, cut across first before slicing. Pour the gravy on top.