Creamy smoked haddock, potato, Cheddar & chive gratin

Cut the potatoes into thin chips, about 1cm x 0.5cm. Put into a large pan of cold water, bring to the boil, then cook gently for 5 minutes until tender. Lift out into a colander with a large slotted spoon and steam dry.

Keep the water in the pan, add the haddock and lower the heat. Poach gently, uncovered, for 5 minutes, then turn off the heat (reserve 75ml poaching water). Meanwhile, melt the butter in a wide ovenproof pan. Add the shallot and a pinch of salt, then cover and cook gently for 5 minutes, stirring occasionally, until soft and starting to caramelise.

Stir the flour into the shallot, then cook for 2-3 minutes until it smells biscuity. With the pan off the heat, gradually stir or whisk in some of the milk until smooth, then add the remainder, plus the reserved poaching water. Simmer gently for 2 minutes, until the consistency of double cream. Preheat the grill to high.

Stir the mustard, ½ the chives and 25g cheese into the creamy sauce. Season, then gently fold in the potatoes. Lift the fish from the poaching water, sit it on top, then scatter the remaining cheese over everything. Grill for 8-10 minutes, until the fish is opaque and flakes easily with a fork, the potatoes are tender and the sauce is bubbling. Scatter with the remaining chives and serve in the dish.