Fish pie is great for leftovers. The nicest is a mixture of fish- say 1/2 white fish (eg cod, haddock) 1/4 salmon and 1/4 smoked fish. If you have poached the fish, reduce the liquid down to concentrate it, and add milk; or just use milk for the sauce. Make a roux of butter and flour, and use this to thicken the sauce. Add chopped parsley to taste, and mix in the flaked fish.
Put the fish and sauce in an ovenproof dish. Put chopped hard boiled eggs and cooked prawns on top of this, then top with mashed potato (or better yet, potato and swede mashed together, which gives a lighter result) Top this with a litttle cheese and some breadcrumbs and bake until golden brown on top.
This probably started out as a way of using up leftovers. You can make it with most types of fish, although probably not really oily fish like herring or mackerel. It can be made with just one fish, but I like a mixture. You can omit the prawns (shrimp in the US? Not sure) but the hard boiled eggs really add to it. Think of it as a sort of fishy shepherd's pie.