Marinate flank steak for at least 4 hours but no longer than 24 hours (see recipe).
In a large zipper-topped plastic bag, combine all ingredients, except beef and nectarines.
Make cuts about 1/2 inch apart and 1/8 inch deep in a diamond pattern in both sides of beef; add to marinade, seal the bag and turn to coat.
Refrigerate, turning occasionally, for at least 4 hours but no longer than 24 hours.
Preheat indoor or outdoor grill to MEDIUM. Remove beef from marinade; reserve marinade. Grill beef, covered, 12 to 14 minutes for medium doneness, brushing occasionally with marinade and turning once.
Add nectarines to the grill for the last 5 minutes of grilling time, turning once and brushing frequently with marinade. Discard any remaining marinade.
Allow beef to rest on a cutting board for 10 to 15 minutes to redistribute its juices then slice across the grain (diagonally). Serve with nectarines.