I don't have a recipe as such, but I got bits of this from the Hairy Bikers and have adapted it over the years. It always gets really good feedback.
I get white cod, smoked haddock, salmon and cooked prawns. I do not buy those 'fish pie mix' packs of fish - they're cut up far too small and have the crap bits. Get good fish.
Poach the fish in milk - start with the cod, then the salmon, then the haddock last as it'll make the milk yellowy. Remove the skins and put the fish into a big oven proof dish, in really big chunks, not chopped up small. Scatter the prawns all over the top.
Sieve the fishy milk to get the bits out. Then make a roux - melt some butter (I use garlic butter if I have some in, otherwise just normal butter is fine) and stir in a big spoonful of flour. Then stir in the fishy milk and cook til thickened. I then finely chop a whole pack of dill and stir in to the white sauce - you need LOTS of dill imo. Then pour this over the fish and gently stir it round, so it's all coated (don't break up the big bits if you can help it).
Then I make mash (I often use the Idaho powedered mash, it's just as nice as homemade) and spoon that over the top and make sure it seals up to the sides of the dish. Bake in the oven for 20 mins or so, then add loads of grated cheese to the top, and put back in the oven for another 15-20 minutes until the sauce is bubbly hot and the cheese is brown.