Spray a large roasting pan with non-stick spray. Place onions in pan; place brisket on top of onions. Rub meat on all sides with garlic, seasonings, jam and lemon juice. Pour cola over and around brisket. Marinate for an hour at room temperature or overnight in the refrigerator.
2. Preheat oven to 325°F. Cook covered. Allow 45 minutes per lb. as the cooking time, until meat is fork tender. Uncover meat for the last hour and baste it occasionally. Remove from oven and cool completely. Refrigerate overnight, if possible. Discard hardened fat which congeals on the surface. Slice brisket thinly across the grain, trimming away any fat. Reheat slices in the defatted pan juices.