Preheat oven to 200°C.
Put the fish into a saucepan with the salt and enough milk or water to cover the fish.
Bring to the boil.
Cover with a lid and stand for 5 minutes.
Flake the fish.
Melt margarine in a sauce pan and sauté onion.
Stir in flour and gradually add milk to the mixture.
Bring to the boil while stirring, cook until sauce thickens.
If the sauce is too thick add more milk.
Mix the fish, eggs, parsley and mushrooms then pour the sauce over and mix lightly.
Pour into a prepared pie dish.
Place pastry over the filling, pinch the edges.
Cut slits into the pastry for the steam to escape.
Decorate the top with leftover pastry.
Brush with milk or egg.
Bake for 35 minutes or until golden brown.