If the arugula has roots attached, cut them off. Fill the sink with cold water and toss the arugula for a few minutes to clean. Spin-dry the leaves and place them in a large bowl.
In a small bowl, whisk together the lemon juice, olive oil, salt, and pepper. Pour enough dressing on the arugula to moisten. Toss well and place the salad on individual plates.
With a very sharp knife or a vegetable peeler, shave the Parmesan into large shards and arrange them on the arugula.