Pre heat oven to 180 degrees C.
Lightly sauté onion in butter, when slightly cooked add flour & mix, slowly add milk until a smooth sauce forms then add chopped parsley & vegetables of your choice.
Lightly toss fish fillets in some olive oil until just turning white.
Add the fish fillets to the sauce mixture and season with salt & pepper.
Roll pre rolled pastry to size of baking dish.
Place fish mixture in a baking dish, lightly brush edges with egg and milk mixture, place pastry on top and cut to size. Press down edges.
Place 4 knife holes on top of pie, brush with egg mixture.
Bake for approx 25mins or until pastry is golden brown.