Tortillas or flatbreads, sour cream, lettuce, tomato
Thoroughly mix together the marinade ingredients in a large baking dish. Lay the flank steak in the marinade and then flip over to coat. Refrigerate for at least 2 hours or overnight is even better.
When ready to cook, take the steak out of the marinade and pat off any excess marinade with paper towel. Heat a tablespoon of oil in a large frypan (or alternatively you can use a grill or BBQ - just make sure it is super hot before placing the steak on the heat). Once it starts smoking, gently place the steak in the pan. Sear for two minutes, then turn over for another 2 minutes. Depending on the thickness of your steak, keep flipping it until the outside is nicely charred and the inside is medium-rare.
Remove the steak from the pan, and allow it to rest for 5 minutes.
While the steak is resting. Mix all of the chimichurri ingredients together except the oil, then whisk the oil last.
Use a very sharp carving knife to carefully slice the steak very thinly (2-3mm thick), across the grain. Lay it out on a platter, spoon the sauce over the top and place in the middle of the table for everyone to dig in.