Put the diced apples in a large mixing bowl.
Stir in the lemon juice.
Chop most of the walnuts roughly, keeping a few halves aside for garnish.
Add the diced celery, cooked chicken breast and chopped walnuts to the apples in the large mixing bowl.
Add the mayonnaise, a bit at a time, until the ingredients are well coated. You may need slightly more or less mayonnaise.
Sprinkle the celery salt and ground pepper over the top of the salad and stir through thoroughly.
Taste the salad for seasoning, adding a bit more pepper if you feel it is needed.
Transfer the Waldorf Salad to a serving bowl and if time allows, refrigerate for about an hour before serving.
Toss the reserved walnut halves over top for garnish.
Sprinkle with parsley before serving, if desired.
Refrigerate leftovers and use within 24 hours.