In a mixing bowl, combine the chopped parsley, oregano, minced garlic, red pepper flakes, olive oil, red wine vinegar, salt, and black pepper. Whisk until well combined.

Place the chicken thighs in a large resealable bag or a shallow dish. Pour half of the chimichurri sauce over the chicken, ensuring it is well coated. Seal the bag or cover the dish and marinate in the refrigerator for at least 30 minutes, or up to 2 hours.

Preheat the grill to medium-high heat. Remove the chicken from the marinade and discard the marinade. Grill the chicken thighs skin-side down for about 6-7 minutes, then flip and grill for another 6-7 minutes, or until the internal temperature reaches 165°F (75°C).

Remove the chicken from the grill and let it rest for a few minutes. Serve with the remaining chimichurri sauce drizzled on top.