Add 3 Tbsp Chimichurri and flank steak to a freezer bag (I love using silicone bags), close, and swoosh around until the steak is well coated with the marinade.
Leave steak on the kitchen counter for up to an hour to marinade and then cook, OR, put in the fridge to marinade for 6-12 hours or overnight and take out of the fridge to bring to room temperature about 1 hour before cooking. Never cook fridge-cold steak as it just burns from the outside and never cooks from the inside.
Cook steak in either a cast iron pan or on the grill.
Preheat cast iron pan or grill to as hot as possible over medium heat. Never preheat over high heat, only medium heat, but for a considerate amount of time to ensure pan/grill is REALLY hot.
Add a little avocado oil to pan if using pan and place steak on it and pan fry or grill for 6-8 minutes on one side without touching of flipping, THEN flip and cook for a further 6-8 minutes on the other side. I cooked mine (shown in photo) to 130 degrees Fahrenheit internal temperature. For bloodier steak cook less time and for more done steak cook longer.
Remove steak from the heat source and place on a cutting board but do NOT, I reapeat: DO NOT, cut into the steak right away. Cover the steak with a stainless steel or glass container or aluminum foil and let steak rest for at least 10 minutes so all juices can reincorporate into the meat.
After 10 minutes cut steak against the grain into thin stripes (see video). Drizzle with Chimichurri and enjoy!