This is a classic "winter warmer" fish pie, made with boneless fillets of firm-fleshed white fish (cod or haddock, for example), but with peeled prawns added for extra interest. It works just as well without the prawns, if you prefer.
Peel the potatoes, and boil in lightly-salted water for about 20 minutes, until they are cooked. Check by pushing a knife into the centre of a potato - it should be soft all the way through.
Drain the potatoes, then add a large knob of butter (or low-fat margerine), a splash of milk and some freshly-ground black pepper. Mash vigorously (!) until the potatoes are smooth and soft - add a little more milk if necessary.
Meanwhile, put a quarter-pint of milk in a wide flat saucepan or frying pan, and add a similar quantity of water. Wash the fish fillets and slip into the milk-and-water - they should just be covered. Bring to the boil, then reduce the heat and allow to simmer for about 10 minutes.
Allow to cool for a minute or two, then drain the milk-and-water into a jug - you will need this to make the sauce. Then, break the cooked fish into flakes with a couple of forks.
Melt an ounce of butter in a saucepan, and pour in half of the liquid used to cook the fish. Separately, mix a heaped tablespoon of cornflour with a little cold water, then pour into the saucepan. I find a teaspoon of English Mustard (the hot, bright yellow version) adds to the flavour. Bring to the boil, stiring continuously with a wooden spoon. The mixture will thicken quickly, so add more of the milk-and-water to get a thick-but-not-solid consistency. Stir hard to avoid lumps!
Add the flaked fish to the sauce, then add 4 ounces of washed, peeled prawns. Stir gently to mix, then pour the whole stodgy fishy sauce into a earthenware oven-proof dish.
Cover the fish sauce with the mashed potato. This is best done with a couple of forks ("... make the forks be with you ..."), a little at a time. Try to get an even cover over the fish, with no gaps. Check carefully around the edge of the dish. The forks can also be used to give an authentic ridged decorative finish to the pie. Decorate the top with a little paprika, and/or a little grated cheese.
Stick the pie dish into a pre-heated oven at 200C (Gas Mark 6) for about 30 minutes. The top should be browned, and both fish and potato very hot all the way through. Serve with microwaved vegetables - I find that carrots and rosemary seems to work well.