1. For vinaigrette, in a small bowl whisk together pear nectar, vinegar, oil, and pepper. Set aside.
2. Cut off and discard upper stalks of fennel, reserving some feathery leaves for garnish (if desired). Remove wilted outer layer of stalks and cut off a thin slice from base. Cut the fennel bulb in half lengthwise. Cut crosswise into thin slices, removing core (if desired).
3. In a medium bowl toss together sliced fennel, arugula, and romaine leaves. Pour about half of the vinaigrette over fennel mixture; toss to coat. Arrange the fennel mixture on 4 salad plates. Top with pears, red onion, and walnuts.
4. Use a vegetable peeler to thinly shave Parmesan cheese. Top the salads with shaved cheese and, if desired, garnish with fennel leaves. Drizzle with the remaining vinaigrette.