In a blender, combine the lemon juice, sugar, onion, mustard and salt. While processing, gradually add oil in a steady stream. Stir in poppy seeds. Transfer to a small pitcher or bowl. Cover and refrigerate for 1 hour or until chilled.
In a large salad bowl, combine the romaine, cheese, cashews, apple, pear and cranberries. Drizzle with dressing and toss to coat.