1 small boiling potato (1/4 lb), peeled and cut into 1/2 inch pieces
1 small onion, finely chopped
2 c chicken broth
1 c heavy cream
4 c coarsely chopped watercress (1 large bunch)
Simmer potato, onion, broth, and cream, uncovered, in a 3- to 4-quart heavy saucepan until vegetables are soft,
stir in watercress and cook mixture, uncovered, over moderately low heat
2 minutes more.
Purée soup in batches in a blender until very smooth, force soup through a
fine mesh sieve into a bowl, discard solids. Season with salt and pepper and let soup stand, uncovered, until cool, 45 minutes, then chill, covered, 1 1/2 hours.