In a small bowl whisk together the garlic, olive oil, vinegar, tarragon, mustard, honey, salt, and pepper. In a large bowl, toss the watercress with half the dressing and divide among 4 to 6 salad plates. Arrange the mushrooms on top of the watercress, drizzle with the remaining dressing, and sprinkle with the hazelnuts. Serve immediately. Serves 4 to 6.