Leek & potato soup

Of the winter soups, leek and potato is one of the most comforting and familiar – and Darina Allen's makes the most of seasonal, local veg

50g butter
450g potatoes, peeled and cut into 1cm pieces (try Golden Wonders or Kerr Pinks)
1 small onion, cut the same size as the potatoes
450g white parts of leeks, sliced (save the green tops for another soup or stock)
850ml-1.2litres/1½-2pts light chicken or vegetable stock
142ml carton whipping cream
125ml full-fat milk
the white part of 1 leek
a small knob of butter
finely chopped chives

Melt the butter in a heavy saucepan. When it foams, add the potatoes, onion and leeks 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 minutes, or until the vegetables are soft but not coloured.
Uncover the pan and discard the paper. Pour in 850ml/11?2 pints of the 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 the cream and all of the milk.
To finish the soup, finely shred the white leek and gently cook it in the hot butter for a few minutes 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.