Season the flank steak with kosher salt and black pepper on both sides.
Preheat your grill or skillet over medium-high heat.
Grill the steak for about 5 minutes on each side for medium-rare, or adjust the time according to your preferred level of doneness.
While the steak is grilling, prepare the chimichurri sauce. In a bowl, combine the chopped parsley, oregano, minced garlic, olive oil, red wine vinegar, and red pepper flakes, if using. Mix well until all ingredients are incorporated.
Once the steak is cooked to your liking, remove it from the heat and let it rest for about 5 minutes. This helps the juices redistribute throughout the meat.
Slice the steak against the grain into thin strips.
Serve the sliced steak with a generous amount of chimichurri sauce drizzled over the top.