Slowly bring a large pan with the water and khombu to the boil. Simmer gently for 2 minutes. Remove the kombu and discard. Add the watercress to the water and leave it to wilt.
Transfer the watercress and liquid to the food processor with the shallots, cucumber, and coriander leaves. Thoroughly blend. Add the lime juice and season with black pepper and sea salt. Refrigerate until chilled.
Note : Greek generals fed watercress to their troops before battle as they believed it enhanced their stamina. In more modern times, it's though to be an excellent blood purifier with anticancer properties.
Makes 4 servings