Boil potatoes until just tender when pierced by a fork. Drain, let cool and dice into a large bowl. Cut onions into quarters and slice thin; add to potatoes along with celery, apples, olives and pickle. In a small bowl, stir mayonnaise, mustard, vinegar and steak sauce. Spoon over potato mixture and mix gently. Season with salt and pepper. Cover and refrigerate for at least 2 hours or until the next day.