Rub the steaks with oil and season with salt and black pepper. Heat a char-grill or bbq and cook the steaks for 2 minutes on each side or until cooked to your liking. Place on a plate, lightly cover with foil and rest the steak for 10 minutes.
Warm the pita pockets following packet instructions and cut in half. Combine mayonnaise and chilli sauce and set aside.
Thinly slice the steaks and fill the pita pockets with celery, cucumber, chilli, tomatoes, watercress and mayonnaise mixture. Sprinkle with sumac and serve.