Heat the oven to 300°F. Put brisket fat-side up in a roasting pan and sprinkle with the onion. Combine all the remaining ingredients in a large measuring cup.
Pour over the brisket. Cover tightly with heavy duty aluminum foil. Bake about 4 hours or until the internal temperature reaches 190°F and meat is tender.
Let the meat rest at least 10 minutes before carving across the grain into thin slices with a sharp knife. Serve with pan drippings.
Even better, cook the brisket a day before serving. Remove the cooked brisket from the drippings. Cover tightly with foil and refrigerate overnight.
Refrigerate the drippings and skim off the hardened fat the next day.
Before serving, slice the brisket while it's cold for a much neater cut. Combine the meat with the pan drippings in a baking dish.
Cover and reheat at 325°F for about 30 minutes or until thoroughly warm.