Heat oil in a nonreactive saucepan and fry the onions until they are lightly browned. Add the remaining ingredients and cook for 2 minutes over low heat, stirring constantly to make sure that the sugar has dissolved.
Allow the marinade to cool and pour it over the chicken.
Cover the bowl with plastic wrap and refrigerate it for 24 hours.
When ready to grill the chicken pieces, remove them from the marinade, drain them, place them on skewers, and grill over hot coals or under the broiler for 10 minutes, turning once, or until the chicken is cooked through.