Step 1: 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.
Step 2: 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.
Step 3: Let the meat rest at least 10 minutes before carving across the grain into thin slices with a sharp knife. Serve with pan drippings.
Step 4: Even better, cook the brisket a day before serving. Remove the cooked brisket from the drippings. Cover tightly with foil and refrigerate overnight.
Step 5: Refrigerate the drippings and skim off the hardened fat the next day.
Step 6: 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.
Step 7: Cover and reheat at 325°F for about 30 minutes or until thoroughly warm.