Place chicken in a large plastic resealable food storage bag. Combine soy sauce, sugar, sherry, ginger, garlic and oil in a small bowl. Mix thoroughly until sugar dissolves. Pour mixture into plastic bag with chicken. Reseal the plastic bag and turn to coat chicken thoroughly. Marinate in refrigerator for 30 minutes to 2 hours. Turn occasionally.
Place onions in boiling water for 4 minutes. Drain the onions and cool in ice water to stop the cooking. Cut off the root ends and snip off outer skins, and set aside.
Drain chicken and reserve the marinade. Weave chicken onto skewers, alternating the chicken with onion and pineapple. Brush with reserved marinade, and discard any remaining marinade.
Grill on uncovered grill over medium hot coals. Heat for 6 to 8 minutes or until chicken is no longer pink in the center, and turn once.