For the vinaigrette: In a bowl, whisk together the walnut oil, olive oil, vinegar, honey, mustard, salt and pepper. Refrigerate until ready to serve.
For the salad: When ready to serve, place the lettuce and tarragon in a large bowl and toss together. Pour the vinaigrette around the edge of the bowl and toss the greens with the dressing. Garnish with the toasted hazelnuts and serve.