This fresh-tasting soup is a great way to appreciate the taste of watercress. Brief cooking and quick chilling are essential to preserve the delicate flavour.
Cut the main stalks from the watercress and reserve. Heat the oil in a pan, add the leek and potato, cover and cook gently for about 10 minutes until soft, without allowing them to colour. Add the vegetable stock, season with salt and pepper and bring to the boil. Simmer for 10 minutes, then add the watercress stalks and simmer for another 5 minutes.