COMBINE cucumbers, sour cream, dill, lemon juice and garlic; cover. Refrigerate until ready to serve.

COOK and stir chicken in 2 Tbsp. of the dressing in large nonstick skillet on medium-high heat 6 to 8 min. or until chicken is lightly browned and cooked through. Remove from skillet; set aside. Add onions and remaining 2 Tbsp. dressing to skillet; cook 5 min. or until tender, stirring occasionally. Return chicken to skillet; cook until heated through, stirring occasionally.

SPOON chicken mixture evenly into pita bread halves. Serve topped with the cucumber mixture.