Cut chicken into 1-1/2-inch pieces; place in large resealable plastic bag. Add zucchini, mushrooms and bell pepper to bag. Combine soy sauce, sherry, sesame oil and garlic in bowl; pour over chicken and vegetables. Close bag securely; turn to coat. Marinate in refrigerator at least 30 minutes or up to 4 hours.
Soak 4 (12-inch) wooden skewers in hot water for 20 minutes.
Drain chicken and vegetables; reserve marinade. Alternately thread chicken and vegetables with onions onto skewers.
Place on medium-hot grill. Brush with half of reserved marinade. Grill for 5 to 6 minutes on each side; brushing with marinade.