In a kettle, cook leeks and onion in the butter over low heat, stirring occasionally, until they are softened. Add the potatoes with the water and the salt. Simmer the mixture, covered, for 30-40 minutes, or until the potatoes are soft. Add milk and h&h and bring the mixture just to a boil, stirring. In a food processor, puree the mixture in batches, and strain it through a very fine sieve into a bowl. Stir the cream and white pepper into the soup and chill, covered, until it is very cold. Garnish with chives and serve cold.
Makes about 11 cups.
A Gourmet Mag. favorite, 1954, by Louis Diat, who was Chef de Cuisine at the NY Ritz-Carlton and a Gourmet contributor. He invented vichyssoise.