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Heat the oven to 200C/180C fan/gas 6 with a baking sheet inside. Heat the butter and oil in a heavy-bottomed pan over a medium-low heat, and cook the onions until golden and soft, about 15-20 mins.
Cut the haddock into 2 x 4cm pieces. Cut the potatoes into wafer-thin slices using a mandoline or sharp knife – if the slices aren’t thin enough, the potatoes won’t become tender while the fish cooks.
Heat the double cream and soured cream in a saucepan large enough to hold all the potatoes. Bring to just under the boil, then add the potatoes and season well. Cook gently for about 10 mins until the potatoes are just slightly softened.
Butter a 2-litre pie dish. Spoon in a third of the potatoes, then scatter over half the onions and a quarter of the cheese. Lay half the fish pieces on top, then season. Sprinkle over another quarter of the cheese, then add another third of the potatoes, then the remaining onions and another quarter of cheese. Add the remaining fish on top, season, then scatter over the remaining potatoes. Pour over any leftover cream mixture, then sprinkle over the remaining cheese.
Carefully slide the dish onto the hot baking sheet in the oven and bake for 40-45 mins, or until golden and the potatoes are tender. If you want to freeze the gratin, leave to cool completely first, then cover tightly. Will keep in the freezer for up to two months. Defrost in the fridge overnight, then reheat in the oven at 180C/160C fan/gas 4 for 20 mins.