How to Make It
In a large saucepan, cover the potatoes with salted water. Bring to a simmer and cook over moderate heat until the potatoes are tender, about 10 minutes. Drain well.
Meanwhile, in a large bowl, mix the mayonnaise with the cocktail onions, capers, tarragon, shallot, vinegar, water and cayenne. Gently fold in the potatoes, celery, and tomatoes and season generously with salt and pepper. Arrange the iceberg lettuce on a serving plate. Scrape the potato salad onto the lettuce and serve.