1. Prepare grill.
2. Sprinkle steak with salt and pepper. 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.
3. 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.