This is a rather rustic soup. For a more refined version, pass it through a food mill before serving.
MELT THE BUTTER IN A SOUP POT and add 1/2 cup water. Add the leeks, sorrel and salt, then cook gently, covered, for 5 minutes over medium-low heat. Add the potatoes and cook another 10 minutes, stirring occasionally, then pour in the rest of the water and gradually bring the soup to a boil. Lower the heat and simmer until the potatoes are tender, about 30 minutes. Once the potatoes are soft, help break them down by pressing them against the side of the pan, making the soup as rough or smooth as you like. Or, if you prefer, pass the soup through a food mill. Taste the soup for salt and add more, if necessary. Serve with freshly ground pepper, a spoonful of creme fraiche swirled in and a delicate sprinkling of chives over the top. If you've got garden chives and they're in bloom, separate the individual flowers at the base and sprinkle on a few along with the green stems.