1. Trim and discard fat from brisket; rinse meat. Lay brisket, fattiest side up, in a 12- by 17-inch roasting pan. Sprinkle onion soup mix, brown sugar, and onions evenly over meat. Pour wine evenly over onions. Cover pan tightly.
2. Bake in a 325º oven until brisket is very tender when pierced, 3 1/2 to 4 hours. With 2 wide spatulas, transfer meat to a platter. Let meat and juices cool about 2 hours. Cover separately and chill until meat is cold, at least 4 hours or up to 2 days.
3. Lift off and discard solid fat from pan juices. Thinly slice brisket across the grain, leaving slices in place; with a wide spatula, transfer neatly to pan with juices. Cover pan tightly with foil.
4. Bake in a 350º oven until meat is hot, 40 to 50 minutes. Serve with pan juices. Add salt to taste.