In a medium bowl, combine the parsley, cilantro, garlic, red pepper flakes, olive oil, red wine vinegar, salt, and black pepper. Mix well to create the chimichurri sauce.
Place the flank steak in a large resealable plastic bag or shallow dish. Pour half of the chimichurri sauce over the steak, reserving the other half for serving. Seal the bag or cover the dish and marinate in the refrigerator for at least 1 hour, or up to 8 hours for more flavor.
Preheat your grill to medium-high heat. Remove the steak from the marinade and discard the marinade.
Grill the steak for about 5-7 minutes on each side for medium-rare, or until it reaches your desired doneness. Use a meat thermometer to check for an internal temperature of 130°F for medium-rare.
Remove the steak from the grill and let it rest for 5-10 minutes before slicing against the grain.
Serve the sliced steak with the reserved chimichurri sauce drizzled on top.