For watercress-flavored mayonnaise take 2 cups watercress leaves and 1 quantity of the above mayonnaise recipe. Place watercress leaves in boiling, salted water. Leave for 1 minute (or until water re-boils) and drain. Place in ice-cold water. Drain thoroughly, pressing out excess water. Blend until it is a smooth puree and either stir into mayonnaise or blend in while mayonnaise is in the processor.