Simmer the chicken in water until it reaches an internal temperature of 160°. Remove from water and dice into small chunks. Place in the refrigerator to cool fully.
Preheat the oven broiler on low and lightly warm the pita.
Once the chicken is cooled fully mix it with the celery, onion, mayo, salt & pepper in a bowl.
Layer the pita with cheese and tomato. Season with salt and pepper, lettuce, and chicken salad. Fold in half. Serve with chips and pickles.