Put the potatoes in a pan of cold, salted water and bring to the boil, then turn off the heat. Drain well. Tip the potatoes back into the pan and pour over the cream, then add the thyme, two-thirds of the parmesan and the haddock. Season and gently mix everything together.

Butter a medium-sized gratin dish, then tip in the potato mixture. Scatter with the remaining parmesan. Will keep covered and chilled for a day or frozen for three months. Defrost fully before cooking.

Heat the oven to 190C/170C fan/gas 5. Bake for 30-35 mins until golden and bubbling around the edges. Leave to cool for at least 5 mins, then serve straight from the dish with a dressed watercress salad on the side, if you like. Scatter with some extra thyme and grind over some black pepper to serve.