Step 1

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.

Step 2

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.

Step 3

Cook over a gentle heat for 10 mins, or until the vegetables are soft but not coloured.

Step 4

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.

Step 5

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.

Step 6

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.

Step 7

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.

Step 8

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.