Remove giblets and neck; rinse turkey with cold water. Pat dry. Tie legs together with string; tuck wingtips under. Combine brown sugar and next 6 ingredients. Rub over turkey. Cover with plastic wrap; chill 8 hours.
Place turkey on a rack in a roasting pan, breast side up. Arrange onion quarters around turkey. Pour 2 cans broth in bottom of pan.
Bake, loosely covered with foil, at 325° for 1 1/2 hours. Uncover and bake 1 1/2 more hours or until meat thermometer registers 180°. (Cover with foil to prevent excessive browning, if necessary.) Remove onion; discard, reserving pan drippings. Let turkey stand 15 minutes before carving.
Combine pan drippings and enough chicken broth to equal 2 cups in a saucepan over medium heat. Whisk in flour, and cook, whisking constantly, 5 minutes or until thickened. Serve with turkey. Garnish, if desired.