Smoked Fish and Potato Gratin

Put the potatoes in a large pan of salted water and simmer until tender. Heat the oven to 200ºC (180º fan) 400ºF, gas 6.

Meanwhile, place the fish in a large, wide pan and pour over the milk and fish stock. Add the thyme then bring to a boil and simmer very gently for about 5 minutes or until the fish just begins to flake. Remove the pan from the heat and strain the fish, reserving the cooking liquid, then break the fish into large chunks. Discard the fish skin.

Melt half the butter in a clean pan and gently cook the leek until soft but not brown. Add the flour and cook for 2 minutes, stirring all the time, then slowly add all but about 125 ml of the fish cooking liquid. Cook gently for 10 minutes then add the parsley and lemon zest, season with salt and pepper and carefully stir in the fish.