Trim and discard most of the surface fat from corned beef 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 corned beef to a platter. Serve hot, warm, or cold; slice meat across the grain.