Trim and discard most of the surface fat from brisket. Rinse meat well under cool running water, rubbing gently to release its corning salt.
Lay meat, fattiest side up, in a 2-inch-deep, 11- by 15- or 16-inch roasting pan. Thinly slice lemon (discard seeds) and onion and lay slices over meat. Sprinkle with peppercorns, allspice, and cloves.
Set pan on middle rack in a 325° oven. Pour about 8 cups boiling water around brisket, seal the pan with foil, and bake until meat is very tender when pierced, about 4 hours. Uncover and drain off all but about 1 cup of the liquid. If desired, reserve the lemon and onion slices and rearrange them on top of the meat.
In a small bowl, mix the mustard and brown sugar; spread evenly over meat, on top of the onion-lemon mixture. Broil about 8 inches from heat until the mustard mixture begins to brown, 3 to 5 minutes. Transfer the brisket to a platter. Serve hot, warm, or cold; slice meat across the grain.
Notes: The weight of the meat shrinks by about half as the brisket cooks.