Nigel Slater’s leek, potato and mackerel

Mackerel served with a velvety soupTrim 4 young leeks, discarding the toughest part of the green and slice them into 3cm rounds. Wash thoroughly under running water to remove grit. Put the sliced leeks in a large pot together with 30g of butter and 3 tbsp of chicken or vegetable stock. Place a piece of baking parchment or greaseproof paper over the top of the leeks, pushing it down into the pan so it touches them. Place a lid on top and cook over a moderate heat for 5 minutes till the leeks have