Cook chicken fillet on a hot grill pan for 8 - 15 minutes ( depending on size of fillet), until nicely charred and cooked through. Cut into large bite size pieces. Fry the peppers and add to chicken, tomato, cucumber and olive oil.
Make dressing by mixing together all ingredients.
To assemble the dish, fill the chicken and topping into the pitta pocket and smother with the creamy mustard dressing. Serve hot or cold.