Peel and dice potatoes and in a bowl combine with water. Discard dark green tops of leeks and chop remaining leeks. In another bowl soak leeks in water to cover, agitating occasionally to dislodge any sand, 5 minutes. Lift leeks out of water and drain in a colander.
In a heavy kettle cook leeks and onion in butter over low heat, stirring occasionally, until leeks are softened. Add potatoes with water and salt and simmer, covered, 40 minutes, or until potatoes are very soft. Add milk and half-and-half or cream and bring just to a boil, stirring.
In a blender puree soup with 1/2 cup chilies in batches until smooth, straining as pureed through a very fine sieve into a large bowl, and stir in sour cream and lime juice. Chill soup, covered, until very cold and up to 1 day.
Cut remaining chilies into small dice. Serve soup garnished with lime slices and diced chilies.
TO ROAST PEPPERS: Using a long-handled fork, char peppers over an open flame or on a rack set over an electric burner, turning, until skins are blackened, 4 to 6 minutes. (Alternatively, broil peppers on rack of a broiler pan under preheated broiler about 2 inches from heat, turning every 5 minutes, 15 to 20 minutes, or until skins are blistered and charred.) Transfer peppers to a bowl and let stand, covered, until cool enough to handle. Keeping peppers whole, peel them, starting at blossom end. Cut off tops and discard seeds and ribs. (Wear rubber gloves when handling chilies.)
Courtesy of Gourmet Magazine