Cut chicken breast into 1 inch pieces and place in a ziploc bag. In a bowl, whisk together remaining ingredients (except onion) and pour into the bag with the chicken. Seal the bag and massage marinade into chicken so that all of it is coated. Place in the refrigerator to marinate overnight. The next day, place chicken and onion on skewers and grill until cooked though. Serve with warm flatbread and Tzatziki sauce (see recipe below). and suggested condiments