Combine cilantro, oregano, garlic, and onion in a food processor and pulse until finely chopped. Slowly add oil and vinegar while pulsing (do not completely purée). Add lime juice and season with salt and red pepper flakes; set aside.
Heat grill to high. Brush steak with oil and lightly season with salt. Grill until nicely charred on one side, 3—5 minutes. Flip and brush cooked side with chimichurri. Continue to cook on other side until medium rare, about 3 additional minutes. Serve with additional chimichurri.
Recipe courtesy of Chef Craig Chasky