Chimichurri Sauce: Add all of the ingredients, except for the olive oil, to a food processor and pulse a few times until chopped. Slowly add the olive oil, while pulsing the mixture a few more times until the olive oil is fully combined. (If necessary, you may need to stop to scrape down the sides of the food processor).
Refrigerate until ready to use. Right before serving, bring it back to room temperature.
Flank Steak: Preheat the grill on high heat for about 10 minutes. Turn down to medium-high right before placing the meat on the grill.
Season both sides of the meat with salt and pepper.
Place the steak on the grill and cook until golden brown and slightly charred, about 4-5 minutes. Turn the steak over and continue to grill for 3-5 minutes for medium-rare (an internal temperature of 135 degrees F), 5 to 7 minutes for medium (140 degrees F) or 8 to 10 minutes for medium-well (150 degrees F).
Transfer the steak to a cutting board, lightly covered with foil, and let rest for the amount of time that you grilled it. Thinly slice the steak across the grain.
Serve as is with the chimichurri sauce, in tacos or even as a taco bowl.