Chilled Potato and Watercress Soup with Caviar Chantilly

• 2 bunches of watercress
• 50 g (2 oz) unsalted butter

Pick the leaves from one bunch of watercress and set aside. Blanch the other bunch and the stalks in boiling water, then drain, refresh in cold water and drain again. Chop finely.

Heat the butter in a pan, add the onion, leeks and potatoes, then cover and sweat over a low heat until softened. Pour in the chicken stock, add the chopped watercress and bring to the boil. Simmer for 20-25 m