Place steak and onion wedges on a grill rack coated with cooking spray. Grill 5 minutes on each side or until desired degree of doneness.
Let stand 5 minutes.
Cut steak diagonally across grain into thin slices.
While steak grills, combine balsamic vinegar and shallots in a small saucepan. Bring to a boil; reduce heat, and simmer 7 minutes or until reduced to 1/4 cup.
Drizzle balsamic mixture over steak slices, and sprinkle with Orange Gremolata.