1. Combine the chicken, salt and a few grindings of black pepper in a large mixing bowl. Stir in the onion or scallions, the celery and the capers.
2. Mix together the mayonnaise and sour cream, and carefully fold it into the chicken mixture. Taste for seasoning.
3. The chicken salad will gain in flavor if it is allowed to rest for an hour or so in the refrigerator before serving.
4. Serve on individual chilled plates on crisp lettuce, or arrange the salad on a large, lettuce-lined platter. In either case, sprinkle the salad with the chopped parsley before serving.
