Preheat the oven to 350 degrees Fahrenheit (175 degrees Celsius).

Rub both sides of the meat with salt, pepper, and if desired, mustard.

Spread half of the onions over bottom of a shallow roasting pan. Place the brisket, fat-side up, in the pan and top with the remaining onions.  Add a quarter cup of water.  Bake, uncovered and basting occasionally, until the meat and onions begin to brown (about one hour).

Add the remaining water and remaining ingredients. (The liquid should never reach more than halfway up the side of the meat.) Cover, reduce the heat to 300°F or place over a low flame, and cook until the meat is fork-tender and the thickest part of the brisket registers about 175°F on a meat thermometer (two to three hours; allow about one hour of cooking time per pound).

Cover the brisket loosely with foil and let stand 20 minutes before carving. Slice the brisket diagonally against grain into slices about an eighth of an inch thick.