Melt 50g butter in a heavy saucepan. When it foams, add 450g potatoes, cut into 1cm cubes, 1 small onion, cut the same as the potatoes, and 450g white parts of leeks, sliced and toss them in the butter until they are well coated.
Season well with salt and freshly ground pepper and toss again. Put a disc of greaseproof paper (called a cartouche by chefs) on top of the vegetables to keep in the steam), then cover the pan with its lid.
Cook over a gentle heat for 10 mins, or until the vegetables are soft but not coloured.
Uncover the pan and discard the paper. Pour in 850ml of the light chicken or vegetable stock, bring to the boil and simmer until the vegetables are just cooked – about 5 minutes. Do not overcook or the soup will lose its fresh flavour.
Purée in a blender until silky smooth, in batches if necessary, then taste and adjust the seasoning. Return the soup to a clean pan and stir in three quarters of a 142ml carton of whipping cream and 125ml full-fat milk.
To finish the soup, finely shred the white part of 1 leek and gently cook it in a small knob of hot butter for a few mins until it is softened but not coloured.
Reheat the soup to a gentle simmer (add some extra stock at this point if the soup is too thick for your liking), then pour into warmed bowls.
Drizzle the remaining cream over each serving, top with a little pile of buttered leeks and a scattering of chives and black pepper and serve at once.
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