Heat the 1 tablespoon butter in a frying pan. Add the chicken and brown over a moderately high heat. Remove the chicken and set aside. Add the onion and celery to the pan. Cook for 2-3 minutes. Add the mushrooms and oregano and cook for a further 3 minutes. Combine the soup mix, water and mustard. Add to the onion mixture. Bring to the boil, stirring constantly, then add the chicken. Simmer the mixture over a low heat for 8-10 minutes or until the chicken is just cooked. Stir occasionally. Remove from the heat. Stir in the cream and parsley, then stand the pan in cold water to cool the mixture. Layer the sheets of filo pastry, brushing each sheet with the melted butter. Cut a 23cm circle from the pastry, then cut the circle into 6 even-sized portions. Turn the chicken mixture into a shallow ovenproof dish, 23 cm in diameter. Place the filo pastry portions on top. Brush with melted butter. Cook in a preheated oven, 190 C, for approximately 20 minutes or until the chicken mixture is thoroughly reheated and the filo pastry is golden brown.