The “classic French” soup was actually invented by French chef Louis Diat around 1910, when he worked at New York’s Ritz-Carlton Hotel. During the Second World War, when the French spa town of Vichy became the capital of the collaborationist government, this soup was served in the United States under a variety of different names. For extra smoothness, push through a sieve after it has been pureed in a food processor.
Melt in a soup pot, over low heat:
      3 tablespoons unsalted butter, or 1 tablespoon butter and 1/4 cup water
Add and cook, stirring, until tender but not browned, about 20 minutes:
      8 large leeks (white part only), cleaned thoroughly and chopped
Stir in:
      3 medium baking potatoes or 2 large baking potatoes, peeled and thinly sliced
      5 cups Chicken Stock or Vegetable Stock, or water
Bring to a boil, reduce the heat, and simmer until the potatoes are soft, about 30 minutes. Puree until smooth. Return to the pot and stir in:
      1/2 to 1 cup heavy cream, or a combination of heavy cream and milk
Season with:
      Salt and ground black pepper to taste
Thin, if necessary, with additional:
      Stock or water
Serve hot or cold and garnish with:
      Snipped fresh chives (optional)