This is from Nigella Lawson’s ” How to Eat”.
Cover the dried porcini with very hot water and leave for 20 minutes or so. Then drain the mushrooms and strain the soaking liquid into the stock. Choose a 2 litre dish in which you will cook (and serve) the fish pie and butter it.
Put the fish in a wide, thick-bottomed frying pan that will take the fish in one layer and cover with milk, the stock with its mushroom liquid and the bay leaves. Bring to a simmer and poach for about 3 minutes. Remove the fish to the buttered dish, and fork into chunks. Sieve the cooking liquid into a jug, reserving the bay leaves.
Melt the butter in a saucepan and add the dried soaked mushrooms, very finely chopped and any grit removed. Fry gently for 2 minutes, stir in the flour and fry gently for another 2 minutes. Off the heat, very slowly add the liquid from the jug, stirring with a wooden spoon or beating with a whisk (whichever suits) as you go.
When all is incorporated, put back on the heat. Add the bay leaves and stir gently until thickened. Pour over the fish in the casserole.
Boil and mash the potatoes with the cream and seasoning now (you want lots of salt and pepper), Preheat the oven to 180C/gas mark 4.
Spread the mashed potato over the fish and season with more nutmeg, pepper and butter (little dots of it here and there) added just before it goes into the oven. Depending on how hot it all is before it goes in the oven, the fish pie should need about 20-40 minutes.