Heat the milk in a saucepan and add the fish. Cook the fish until it flakes easily. Once cooked, remove the fish but keep the milk in the pan.
Meanwhile, boil the potatoes until tender.
Follow the instructions on the parsley sauce packet to make the parsley sauce. Sieve the milk that you cooked the fish in and use that to make up the parsley sauce (using the amount stated on the packet).
Add the leek, onion and cream to the sauce. Cook for 2 to 3 minutes. Add flour to thicken. Add lemon juice and salt and pepper to taste.
Place fish evenly at the bottom of the ovenproof dish, then pour sauce on top of it and spread evenly with one handful of grated cheese on top.
Mash the potatoes using milk and butter, season with salt and pepper, then spread the mash on top of the fish. Top with the remaining handful of grated cheese.
Cook in oven for 20 minutes until golden. Enjoy :-)