Combine first 6 ingredients in a small bowl. Score a diamond pattern on steak. Place steak in a shallow dish. Spread green onion mixture on steak. Cover and marinate in refrigerator at least 8 hours.
Place steak on grill rack coated with cooking spray; grill 6 minutes on each side or to desired degree of doneness. Let stand 5 minutes. Cut steak diagonally across grain into thin slices. Serve with Tomato-Avocado Salsa. Garnish with cilantro sprigs, if desired.