Cut the celery into very thin strips.
Cut the apples into small cubes, and halve the grapes.
Mix drained yogurt, mustard, powdered sugar, salt, and pepper together.
To avoid thinning the cream, the Waldorf salad should be mixed right before serving.
Toss the cream with the celery, apples, and 2/3 of the grapes.
Garnish the salad with the remaining grapes and walnuts.
Cut the capon into legs and breasts, and serve with potatoes, sauce, and Waldorf salad.