Watercress and Buttermilk Vichyssoise

First of all, melt the butter in a large (6 pint/3.5 litre) saucepan, then add the prepared leeks, onion, potatoes and half of the watercress.

Stir them around so that they're coated with the melted butter. Next, sprinkle in some salt, then cover with a lid and let the vegetables sweat over a very gentle heat for about 20 minutes, giving the mixture a good stir about halfway through.

After that, add the stock, bring everything up to simmering point and simmer, covered, for about 10-15 minutes or until the vegetables are quite tender. Then remove the pan from the heat and leave the soup to cool a little, then liquidise until it is smooth – you'll need to do this in batches.

A bowl to put the first batch in is useful. Next, transfer the soup to a large bowl and leave until cold. When the soup is cold, stir in three-quarters of the buttermilk and season to taste. Cover the bowl with clingfilm and chill the soup thoroughly before serving – overnight is ideal.

When you are ready to serve the soup, put the other half of the watercress in a large bowl. Then, using the end of a rolling pin or a pestle, ‘bruise’ the leaves – this will release the oil in the leaves. Now whiz the watercress in the blender with about a third of the chilled soup (remembering to reserve a few leaves for garnishing) and when it is smooth, stir it into the rest of the soup.

Serve the soup in bowls that have been very well chilled and garnish each one with a swirl of the remaining buttermilk and a few reserved watercress leaves.