In a medium bowl, combine the chopped parsley, minced garlic, oregano, red wine vinegar, red pepper flakes, and olive oil. Stir well and set aside for at least 20 minutes to allow flavors to meld.
Preheat a grill or cast-iron skillet over high heat.
Pat the flank steak dry with paper towels and season generously with salt and black pepper on both sides.
Place the steak on the grill or skillet and cook for 5 to 7 minutes per side, or until an internal temperature of 130°F (54°C) is reached for medium-rare.
Remove the steak from the heat and transfer to a cutting board. Tent loosely with foil and rest for 10 minutes.
Slice the steak against the grain into thin strips.
Arrange the steak on a platter and spoon the chimichurri sauce liberally over the meat before serving.