In a steamer set over boiling water steam the potatoes, cut into 1/2-inch dice, covered, for 7 to 10 minutes, or until they are tender, and let them cool. In a large bowl combine the apples, cut into 1.2-inch dice, with the potatoes, the celery, and the scallion. In a small bowl whisk together the lemon juice, the mustard, the sugar, the parsley, the cream, the salt, and freshly ground black pepper to taste. Pour the dressing over the potato mixture and toss the salad gently but thoroughly.