Combine parsley, onion, garlic, vinegar, oil, pepper flakes, and salt and pepper to taste in a small bowl. Season steak with salt and pepper to taste.
To Freeze: Freeze steak and sauce separately.
To Reheat: Grill steak over very hot fire until browned on both sides and slightly less done than you want, about 10 mins. Place steak in a baking dish and coat with parsley sauce. Cover dish with foil and let rest for 5 minutes. Serve with remaining sauce.