Combine sugar, lemon juice, onion, mustard, and salt in a food processor or blender; 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.

Toss romaine lettuce, shredded Swiss cheese, cashews, dried cranberries, apple, and pear together in a large serving bowl. Pour dressing over salad just before serving, and toss to coat.