Wash and dry the fish thoroughly, remove all skin and bones and cut into neat pieces. Place in layers in a pie-dish, sprinkling each layer with salt, pepper, onion, herbs and parsley. Add water to ¾ fill the dish and mix with it the vinegar. Cover the pie-dish with pastry and bake in a very hot oven (450° F., Gas 8) until pastry is set, then reduce heat to moderate (350° F., Gas 4).
Serve either hot or cold.