This is one of my favourite chilled soups. The cheese adds a little tanginess which blends well with the leek and peppery watercress.
Sweat the onion and leeks in the butter for about 5 minutes, until beginning to wilt. Add the potatoes and cook for 5 minutes longer. Add the stock and bring to the boil, then reduce the heat to a simmer and cook for 25 minutes, until all the vegetables are soft. Remove from the heat. Add the leaves from the bunch of watercress and leave to infuse in the pan for 5 minutes, then blitz to a smooth purée in a blender.
Put the cream, milk and Roquefort in a small pan and heat gently, stirring until smooth. Add to the soup, stir well together and add seasoning. Strain through a fine sieve, then chill until ready to serve.