MIX all ingredients except chicken, onions and sumac in large bowl. Add chicken; toss to coat. Refrigerate 15 min. to marinate.
HEAT greased barbecue to medium-high heat. Remove chicken from marinade; discard marinade. Thread chicken alternately with onions onto 8 skewers.
GRILL 16 to 18 min. or until chicken is done, turning occasionally. Sprinkle with sumac.
serving size = 2 skewers (122 g)