From the Cook's Illustrated website: Lentils du Puy, sometimes called French green lentils, are our first choice for this recipe, but brown, black, or regular green lentils are fine, too. Note that cooking times will vary depending on the type of lentils used. Lentils lose flavor with age, and because most packaged lentils do not have expiration dates, try to buy them from a store that specializes in natural foods and grains. Before use, rinse and then carefully sort through the lentils to remove small stones and pebbles. The soup can be made in advance. After adding the vinegar in step 2, cool the soup to room temperature and refrigerate it in an airtight container for up to 2 days. To serve, heat it over medium-low until hot, then stir in the parsley.