Pre heat your oven to gas mark 4 or equivalent. Peel and prep your potatoes and get them on to boil. When cooked, drain thoroughly, mash with half the butter and season with salt and pepper.
Grab yourself an adequately sized oven proof dish, place the frozen fillets into the dish and cover with all the milk. Place into the oven and leave for half an hour. Uncovered.
In the meantime, gently melt the other half of the butter and whisk in the flour to make a roux.
Grab yourself a large glass bowl and a sieve to drain the fish from the milk, once it has cooked for 30 minutes. Place the fish back in the dish, that you've cooked it in and pull the meat away from the skin, making sure to dispose of the skin. Gradually add the milk to the roux, whisking all the while, add three quarters of the cheese along with a well heaped teaspoon of parsley and season as you go.
Pour the sauce over the fish, top off with mash and the rest of the cheese. Place back in the oven, gas mark 4, for an additional half an hour.