Combine parsley, garlic, and oregano in a bowl. Whisk in oil, vinegar, salt, and pepper.
Preheat broiler. Slice steak into 4 pieces and lay on a roasting pan or baking sheet. Massage both sides of steak with parsley mixture. Broil 3 to 4 minutes per side. Take steak out of oven, cover, and let stand 5 minutes. Serve the steak with the raw onions.