Place beef in a 15x10x1-in. baking pan; rub with brown sugar. Refrigerate, covered, 24 hours.
In a small bowl, mix salt, chopped onion, bay leaves and seasonings; rub over beef. Refrigerate, covered, 3 days, turning and rubbing salt mixture into beef once each day.
Preheat oven to 325°. Remove and discard salt mixture. Place beef, onions, carrots, celery and stout in a roasting pan. Add water to come halfway up the brisket. Roast, covered, 4-4-1/2 hours or until meat is tender. Cool meat in cooking juices for 1 hour.
Remove beef; discard vegetables and cooking juices. Transfer beef to a 13×9-in. baking dish. Refrigerate, covered, overnight.
Cut diagonally across the grain into thin slices. Serve with rye bread, cheese and mustard.
