Place the flank steak in a shallow dish or large resealable bag.
In a small bowl, whisk together the chimichurri sauce, lime juice, salt, pepper, and garlic powder. Pour the mixture over the steak and turn to coat evenly.
Cover or seal and marinate in the refrigerator for at least 30 minutes or up to 8 hours.
When ready to cook, remove the steak from the refrigerator and let it sit at room temperature for 15 minutes.
Preheat a grill or large skillet to medium-high heat. Lightly oil the grates or pan.
Remove the steak from the marinade, allowing excess to drip off. Cook for 4–5 minutes per side for medium doneness, or until the steak reaches your desired internal temperature.
Transfer the steak to a cutting board and let rest for 5–10 minutes.
Slice thinly against the grain and drizzle with additional chimichurri sauce before serving.