1 cup finely chopped white and pale green part of leek, washed well
1/2 cup finely chopped onion
2 tablespoons unsalted butter
1 pound boiling potatoes
4 cups chicken broth
2 cups water
1/2 pound fresh sorrel, stems discarded and the leaves rinsed, spun dry, and shredded coarse (about 8 cups)
1/2 cup heavy cream
1/4 cup snipped fresh chives, or to taste, plus, if desired, additional for garnish
In a large saucepan cook the leek and the onion with salt and pepper to taste in the butter over moderately low heat, stirring, until the vegetables are softened, add the potatoes, peeled and cut into 1-inch pieces, the broth, and the water, and simmer the mixture, covered, for 10 to 15 minutes, or until the potatoes are very tender. Stir in the sorrel and simmer the mixture for 1 minute. Purée the mixture in a blender in batches, transferring it as it is puréed to a bowl, and let it cool. Stir in the cream, the chives, and salt and pepper to taste, chill the soup, covered, for at least 4 hours or overnight, and serve it sprinkled with the additional chives. Also good warm.