- 1 small boiling potato, peeled and cut into 1/2-inch pieces
- 1 small onion, finely chopped
- 2 cups low-sodium chicken broth
- 1 cup heavy cream
- 4 cups coarsely chopped watercress
- Simmer potato, onion, broth, and cream, uncovered, in a 3- to 4-quart heavy saucepan until vegetables are soft, about 10 minutes. Stir in watercress and cook mixture, uncovered, over moderately low heat 3 minutes more.
- Puree soup in 2 batches in a blender until very smooth, about 45 seconds per batch (use caution when blending hot liquids). Force soup through a fine-mesh sieve into a bowl, pressing hard on solids and then discarding them. Season with salt and pepper and let soup stand, uncovered, until cool, 45 minutes, then chill, covered, 1 1/2 hours.