Preheat a grill pan or light a grill.
Season with salt and pepper the flank steak.
Using a grill pan brush a shadow of oil on the bottom of it. Place the pan over high heat and when hot add the flank steak. Cook until nicely charred outside and medium rare inside. About 3 minutes per side.
Transfer the steak to a carving board and let rest for 5 minutes.
Thinly slice the steak across the grain and serve with the herbed salsa.