Mix the first 5 ingredients; rub over brisket. Mix water, chili sauce, soy sauce and syrup; pour 2-1/2 cups into a bowl or shallow dish. Add brisket and turn to coat. Cover and refrigerate at least 8 hours or overnight. Cover and refrigerate remaining marinade.
Preheat oven to 350°. Place onions in an ovenproof Dutch oven. Drain beef, discarding marinade in bowl. Place brisket over onions; arrange potatoes around meat. Pour remaining marinade over the top.
Bake, covered, until meat is tender, 2-1/2 to 3 hours. Remove brisket and potatoes to a serving plate. Skim fat from cooking juices. Cut diagonally across the grain into thin slices. Serve meat and potatoes with cooking juices. Freeze option: Using a slotted spoon, remove vegetables to a bowl to cool. Transfer whole brisket and cooking juices to a greased 13x9-in. dish; cool completely. Add vegetables. Cover and freeze. To use, partially thaw in refrigerator overnight. Remove from refrigerator 30 minutes before baking. Preheat oven to 350°. Reheat brisket and vegetables, covered, until a thermometer inserted in center reads 165°, about 1 hour. Thinly slice meat across the grain. Serve meat and potatoes with cooking juices.