In a blender or food processor, combine sugar, lemon juice, onion, mustard and salt. Process until well blended. With machine still running add oil in a slow steady stream until mixture is thick and smooth. Add poppy seeds and process just a few seconds more to mix.
In a large serving bowl combine the romaine lettuce, shredded Swiss cheese, cashews, dried cranberries, cubed apple and cubed pear. Toss to mix then pour dressing over salad just before serving and toss to coat.