Preheat oven to 325 F. Combine 2 Tbs brown sugar and Cajun seasoning; rub over both sides of brisket. Combine remaining sugar with tomatoes and their juice, ketchup and vinegar; reserve.
In flameproof roasting pan heat oil over medium-high heat. Add brisket; cook, turning once until browned, 2-3 minutes per side. Sprinkle onion around brisket. Pour tomato mixture over meat; cover with foil. Bake until very tender, 2 1/2-3hours.
Transfer brisket to serving platter. Place pan with juice over two burners on high heat. Bring to boil; cook, stirring until reduced by 1/4, about 8 minutes. Serve over brisket.