This cold leek-and-potato soup, a French-American classic, was perfected in the early 1900s by Louis Diat, the chef at the Ritz Carlton Hotel in New York. Everyone has eaten it (or at least heard of it), but if it is served at all these days, it is often a watery, grainy, yellow-green puree, instead of the ivory-colored velvety cream it should be. Diat's soup is basically milk and cream that is flavored and thickened by the vegetable puree; most current recipes are just the reverse, a not-very-smooth puree with a token amount of cream tossed in. Diat included "medium" cream, now a thing of the past; the recipe below substitutes more milk and heavy cream, but scrupulously follows Diat's directions for such flavor-enhancing steps as sauteing the leeks in butter. Made correctly, there is no reason to be bored with this soup, and it doesn't need jazzing up with a lot of "creative" ingredients (or even a sprinkling of chives). It fully deserves its fame.