Watercress adds a peppery note to the soup; look for “live” watercress, which is packaged with its roots attached. It stays fresher longer and is easier to clean. To prep it, trim off and discard the roots, rinse and drain the greens, then cut them into 1½-inch lengths, discarding any stems that are thick or tough. If you prefer, substitute an equal amount of baby spinach for the watercress, but roughly chop the leaves before using. We also liked this soup made with chicken bouillon paste instead of chicken broth; use 2 tablespoons of paste dissolved in 2 quarts of water.