1Heat gas or charcoal grill. Using sharp knife, cut tenderloin in half horizontally, cutting to, but not through, other side; open flat as you would a book.
2Carefully brush oil on grill rack. Place nectarine halves, cut sides down, and pork on grill over medium heat. Cover grill; cook 8 to 10 minutes, turning once, until nectarines are thoroughly heated and pork has slight blush of pink in center. Remove nectarines and pork from grill. Cover pork; let stand 5 minutes.
3Cut nectarine halves into slices. Thinly slice pork. In large bowl, gently toss spinach and dressing.
4On each of 6 plates, evenly divide spinach, nectarine slices, pork slices and cheese. Sprinkle with pepper, if desired.