Step 1

Preheat the oven to 200ºC, gas mark 6 and put a baking sheet in. Heat 2 tbsp olive oil in a large frying pan and fry the onions over a medium to low heat until beginning to soften, about 5 minutes. Season, then add 50ml water. Cover the pan and sweat the onions over a low heat until completely soft, about another 5 minutes. Add the garlic and ½ the paprika to the pan and cook for another 4 minutes or so.

Step 2

Put the double cream, soured cream and milk into a large saucepan. Season, then bring to a gentle simmer. Using a mandolin, finely slice the potatoes. They must be wafer-thin. As you cut the slices, drop batches into the pan with the cream. Once the potatoes have been sliced, add the last batch and part-cover the pan with a lid, leaving a little gap at the side.

Step 3

Cook the potatoes for 25-30 minutes, until tender but with a little resistance when you put the point of a knife into them. Turn them over occasionally and make sure they don’t burn at the bottom.

Step 4

While the potatoes are cooking, clean and slice the mushrooms. Heat the remaining olive oil in a frying pan and briskly fry the mushrooms to get a good colour. Season and add the remaining paprika. The mushrooms will exude a lot of water over time – keep cooking until the liquid evaporates. Add ½ the butter to the mushrooms – it helps the flavour – and butter a 2L gratin dish with the rest.

Step 5

Cut the fish across the loin into 2cm slices. Spoon ½ the potatoes into the buttered dish and sprinkle with ¼ of the dill. Put ½ the onions and ½ the mushrooms on top, then the fish. Season and sprinkle with ¼ more dill. Add the remaining onions and mushrooms. Sprinkle with more dill (reserve some for the top) and spoon on the rest of the potatoes. This will be the top of the gratin, but it doesn’t have to be neat.

Step 6

Slide the dish onto the hot baking sheet. Cook for 30 minutes, or until the liquid has completely reduced, the potatoes are soft and the top is golden. Let the dish cool for about 8-10 minutes – it will be very hot – then scatter over the dill. Serve with lemon wedges and the beetroot salad (see link under the title) on the side.