How to Make It
In a large saucepan, combine the leeks and potatoes with 6 cups of the water. Bring to a boil, season with salt and simmer over moderately low heat, partially covered, until the potatoes are tender, 20 to 30 minutes. In a blender, puree the soup in batches until very smooth. Return it to the pan and, if needed, add up to 1 cup of water to loosen. Add the cream and season with salt. Warm over moderate heat until heated through. Ladle into bowls and serve.