1. Combine parsley, onion, garlic, vinegar, oil, pepper flakes, and salt and pepper to taste in small bowl. Season steak with salt and pepper to taste.
2. Grill steak over very hot fire until browned on both sides and slightly less done than you want, about 10 minutes.
3. Place steak in baking dish and coat with parsley sauce. Cover dish with foil and let rest for 5 minutes. Slice steak thinly against grain and serve with sauce remaining in baking dish.
Great sliced on salads or served with fresh summer corn on the cob and marinated vegetables.