Start by washing the bundle of flat leaf parsley and removing the stems. Place the parsley, garlic, red onion, lemon juice, red wine vinegar, olive oil, pepper flakes, salt, and pepper into a food processor.
Blend the chimichurri ingredients together until you have a coarse mixture.
Separate ¼ cup of chimichurri to be used for marinade. Save the remainder to use a finishing sauce.
Place meat into a food storage bag. Add the chimichurri marinade. 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.
Heat your grill or cast iron pan to a medium-high heat. Place the marinated flank steak flat on the grill.
Let the meat cook for about seven minutes.
Then, turn the steak over and cook the other side for another seven minutes.
Once the steak has reached the desired temperature, between 130° and 145° at the center, remove it from the heat and cover with foil.
Let meat rest for 10 minutes.
Cut the rested Flank Steak against the grain into one inch strips. Add some extra chimichurri sauce to the meat and serve.