Sprinkle spices over brisket. Spread with onions. Bake uncovered at 350 degrees F. for one hour. Pour in boiled water, cover pot and simmer at 300 degrees F. for an additional two hours. Mix potato starch with water until thoroughly dissolved. Remove brisket and onions from pan. Over medium heat pour cornstarch mixture into pan juices, stirring constantly until mixture thickens. Slice brisket and place in gravy.