COMBINE first 5 ingredients. Refrigerate until ready to serve.
COOK and stir chicken in 2 Tbsp. dressing in large nonstick skillet on medium-high heat 6 to 8 min. or until chicken is done. Remove from skillet; set aside. Add onions and remaining dressing to skillet; cook 5 min. or until onions are crisp-tender, stirring occasionally. Return chicken to skillet; cook 2 to 3 min. or until heated through, stirring occasionally.
SPOON chicken mixture into pita bread halves. Serve topped with cucumber mixture.