Ingredients
2 fillets of any firm white fish, such as pollack, cod or haddock (about 600g/1lb 5oz in total)
1 small fillet of undyed smoked haddock (about 175g/6oz)
750ml/1¼ pints milk
1 medium onion, roughly chopped
1 large carrot, roughly chopped
1 stalk celery, roughly chopped
1 bay leaf
bunch fresh parsley
a few peppercorns
1kg/2lb 4oz floury potatoes, such as Maris Piper or Desirée
125g/4½oz butter, plus extra to grease the dish and dot on top of the pie
75g/2½oz plain flour
salt and freshly ground black pepper
2 handfuls cooked peeled prawns
buttered minted peas, to serve
crusty bread, to serve

Method
1. Place the fillets of fish in a medium saucepan. Add the milk, onion, carrot, celery, bay leaf, a couple of stalks of parsley and the peppercorns.
2. Place the pan on a low heat and let the milk heat up gently. As soon as it comes to a simmer, switch off the heat and cover the pan. The fish will continue cooking in the hot milk.
3. Meanwhile, peel the potatoes, cut them into even, bite-sized chunks and put them in a large pan. Add just enough water to cover and put the pan on the hob over a high heat. Add a teaspoon of salt and let the water come to the boil. Lower the heat to a simmer and cook the potatoes until they are just tender.
4. Carefully drain the potatoes and allow them to cool in a colander for a minute or two. Return them to the pan and mash them, adding 50g/2oz of the butter, cut into cubes.
5. Stand a sieve over a large jug and tip in the fish and milk mixture. Wash the pan in which the fish was cooked and dry it well.
6. Add 3-4 tablespoons of the fishy milk to the mash and stir it in well. Add some freshly ground black pepper, taste the mash, and add some salt if you think it needs it. Put the mash to one side.
7. Heat the oven to 200C/400F/Gas 6.
8. To make the béchamel sauce, put the remaining 75g/2½oz butter in the clean pan and melt it over a low to medium heat. Add the flour and stir well with a wooden spoon to make a roux. Cook for two minutes, stirring every few seconds. Then gently whisk in one third of the hot fishy milk. The paste will quickly turn into a very thick sauce. Add another third of the milk, whisking all the time, and then the final third, so you end up with a creamy sauce. Season the béchamel with salt and freshly ground black pepper, turn the heat down to very low, and let the sauce bubble gently for five minutes while you prepare the fish.
9. Remove the vegetables, herbs and peppercorns from the fish and discard. Carefully pick up a chunk of fish. Peel off any skin and discard, then gently feel the flesh between your fingers for bones, being careful not to over-shred the fish. Put the boneless fish on a clean plate.
10. Turn off the heat under the béchamel and add the fish to the sauce. Add the prawns, then chop the remaining parsley and stir this in too. Taste the sauce once more and add more seasoning, to taste.
11. Generously butter a pie dish and pour in the fishy béchamel. Spoon over the mash and spread it carefully across the surface of the fish sauce. Dot a little extra butter over the top of the pie.
12. Wearing oven gloves, put the pie in the oven and bake for about 25 minutes or until the top is starting to brown and the fishy sauce is bubbling up the sides of the mash.
13. Serve with buttered minted peas and crusty bread to mop up the sauce.