Place the fennel, lettuce, tarragon, and scallions in a large bowl. Whisk together the lemon juice, mustard, salt, and pepper in a small bowl. Whisking constantly, slowly add the oil. Pour the vinaigrette over the salad and toss. Top with the Pecorino (if using).
In Advance: Assemble the greens and make the vinaigrette. Refrigerate them separately for up to several hours. Combine just before serving.