Hopefully one of your countrymen or -women will respond. I've never had fish pie and don't remember anyone I know having it, either. Apparently it's one of those things that didn't make it across the pond
Lots of recipes on Google - some bad, some good. Fish "pie" varies from boring nursery food to good enough for a dinner party. I hope mine will be the latter.
I'm including giant prawns (shrimp, to you), cod and salmon.
The fish is poached lightly in the milk for the sauce with the chopped dill, lemon zest and salt and pepper before the sauce is made The sauce will have leeks, butter, Channel Islands milk (from Jersey and Guernsey cows), dill, a little mustard, salt and pepper, lemon zest and juice.
For the sauce melt butter then sweat the chopped leaks in it until soft but not coloured then add flour and cook off adding the warm milk as in béchamel sauce and the other sauce ingredients to taste. Then the fish, the uncooked frozen peas and the sauce will be placed in a deepish dish.
The potatoes are boiled then drained (I like to put them back in the pan and shake them over the heat to dry them) and mashed with butter and a little warm milk, ground black pepper, salt if needed and spread on the fish mixture (you can pipe it if you want to be "posh"). Pop in the oven at gas mark 6 (if you're in UK) or 200C/400F for about 20 minutes or until bubbling and lightly browned on top.
Normally I'd put cheese (usually cheddar) in the sauce but my cousin doesn't like it and I like my cousin so it's left out here.
It freezes well before the oven baking and I often do it in individual dishes to freeze so I have something to fish out of the freezer when I haven't time to fiddle with cooking. I sometimes add frozen chopped spinach, well drained and squeezed) to the sauce but haven't this time.
This version is fairly rich with the Channel Islands milk and butter but you wouldn't eat it everyday and if you have it every Friday you'd probably make a less rich version but this is Good Friday and I haven't seen Jeannine and Fred since before Christmas as neither of them are able to drive now. I go to see them and we take it in turns to cook lunch.