Preheat grill to 350-400 degrees, F. (medium-high) heat. Cut off tips of wings and break joints to make chickens lie flat on grill. Sprinkle chicken with 1/2 t. salt and 1/2 t. pepper.
Meanwhile, combine onion, next 6 ingredients and remaining 1/2 t. salt and 1/2 t. pepper in a small saucepan; cook over medium heat, shirring occasionally, until butter is melted and mixture comes to a boil. Remove from heat; reserve 1 cup sauce. Place chicken skin side down, on grill. Grill, covered with grill lid, 15 mintues; turn and grill 15 to 20 mnutes or until done. Baste with ramining sauce during last 5 minutes of cook time. Serve with reserved sauce