To a gallon sized ziploc bag, add all of the marinade ingredients and whisk to combine.
Add the flank steak to the marinade, make sure that it’s fully covered in the marinade, and seal trying to remove as much air as possible. Marinate in the fridge for at least 2 hours and up to 8 hours.
Preheat the smoker to 225°F/110°C for 15 minutes, place the steak on the smoker and smoke for 45 minutes then set aside.
Meanwhile, as the steak is on the smoker, make the chimichurri sauce: In a food processor, combine all of the ingredients except for the olive oil. Pulse until finely chopped NOT pureed, alternatively chop by hand with a sharp knife. Remove onto a bowl, add olive oil and gently stir. Allow to sit for an hour before using.
Increase the smoker temperature to 450°F/230°C, and sear the steak for 3 minutes per side. The internal temperature of the steak must be 135°F/57°C for medium rare.
Remove from the smoker and allow to rest for 10 minutes before slicing against the grain. Top with chimichurri sauce
Notes
Cook to temperature. I give general guidelines about timing when smoking your beef, but it’s always best to use a meat thermometer for an accurate way to check the internal temperature.
Remove the meat from the smoker when it's 5 – 10 degrees away from the desired temperature. It will continue to cook as it rests and will end up at the perfect temperature.
Flank steak is at its most tender when cooked to medium-rare or medium. If you cook it for longer it won’t be quite as juicy.