Parboil the watercress in boiling water for 10 seconds; drain, squeeze dry, and finely chop.
Whisk the egg yolk in a mixing bowl for 2 minutes. Slowly drizzle in the oil, still whisking.
Add the lemon juice, shallot, capers, cornichons, tarragon, cooked egg, mustard, and blanched watercress. Stir well. Season with salt and white pepper to taste.
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