Start by washing the bundle of fresh parsley and removing the stems. Place the parsley, fresh garlic cloves, red onion, lemon juice, red wine vinegar, olive oil, red pepper flakes, salt, and pepper into the bowl of a food processor.
Blend the chimichurri ingredients together until you have a coarse mixture.
Separate ¼ cup of chimichurri to be used for steak marinade. Place the remainder in a small bowl to use as a finishing sauce.
Place steak into a food storage bag. Add the chimichurri marinade. Then, close the bag and massage the marinade into the meat. Let the steak marinade for at least 30 minutes, but no more than 2 hours. (If you are going to use the steak within 30 minutes, leave it on the counter so that the steak will be room temperature when you start cooking.)
Heat your grill or cast iron pan to a medium-high heat. Place the marinated flank steak flat on the preheated grill. (Now leave it alone!)
Let the meat cook for about 5-7 minutes on the first side.
Then, turn the steak over and cook the other side for another 5-7 minutes. (The meat should come cleanly away from the grill and have started to brown.)
Use a meat thermometer to check for the desired internal temperature. Then, remove it from the heat and cover with foil.
Let the steak rest for 10 minutes. If you like your steak more rare, take it off the grill a few degrees lower than the desired doneness because the temperature will rise 5+ degrees while it rests.
Cut the rested Flank Steak against the grain into one inch strips. Add some extra chimichurri sauce to the meat and serve.