Notes: If you use buttermilk, this soup will have a pleasant tang; cream, on the other hand, brings out the flavor of the watercress and potato. Serve soup hot or cold.
How to Make It
Step 1
Peel potatoes and cut into 3/4-inch chunks. In a 3- to 4-quart pan over high heat, bring potatoes and broth to a boil; cover, reduce heat, and simmer until potatoes are tender when pierced, 18 to 25 minutes.
Step 2
Meanwhile, rinse and drain watercress. Pick six tender sprigs from stems and reserve. Coarsely chop remaining watercress, including stems; you should have 2 1/2 to 3 cups. Stir chopped watercress into potato-broth mixture and cook just until watercress is bright green and wilted, about 1 minute.
Step 3
Pour half the potato-watercress mixture into a blender and, holding lid closed with a towel, whirl until smooth. Pour into a bowl. Repeat to purée remaining soup; pour into bowl. Stir in buttermilk.
Step 4
To serve soup hot, return to pan and stir over medium heat just until steaming (do not boil). To serve cold, cover and chill until cold, 3 to 4 hours.
Step 5
Ladle into bowls. Add salt and pepper to taste. Garnish with reserved watercress sprigs.
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