How to Make It
Preheat the oven to 350°. Spread the hazelnuts in a pie plate and toast for about 10 minutes, or until fragrant. Let the nuts cool slightly, then coarsely chop.
Meanwhile, peel the tangerines, removing all the bitter white pith; if using oranges, peel with a sharp knife. Working over a large bowl, cut in between the membranes to release the citrus sections.
In a bowl, whisk the sherry vinegar with the balsamic vinegar, honey, shallot and thyme. Whisk in the olive oil, hazelnut oil and truffle oil in a slow stream. Season the dressing with salt and pepper.
Add the watercress and parsley to the tangerine sections and toss. Add 1/3 cup of the dressing and toss well. Arrange the salad on a large platter; arrange the prosciutto decoratively around the salad. Sprinkle with the toasted hazelnuts and serve with the remaining dressing.