Set aside to let cool completely, about 45 minutes.
, peeled and sliced 1/3 inch thick.watercress, coarsely chopped.
European cucumber.Add the leek and cook over moderate heat, stirring, until softened, 3 minutes.Add the broth and water and bring to a boil.
Add the potato, cover and simmer over low heat until tender, about 15 minutes.Add the watercress and cook until bright green, about 2 minutes..
Working in batches, puree the soup in a blender.
Return the soup to the saucepan.Cook, whisking constantly, 1 minute.
Pour mixture into an 8 1/2- x 4 1/2-inch loaf pan.Use a spoon to skim off any bubbles from surface.
Let cool completely at room temperature, uncovered and undisturbed, until set, about 45 minutes.Run a knife along edges of pan.