In a medium bowl, mix together the chopped parsley, cilantro, minced garlic, red pepper flakes, salt, and black pepper. Slowly whisk in the olive oil and red wine vinegar until well combined. Adjust taste with lime juice.

Place chicken thighs in a resealable bag or bowl. Pour half of the chimichurri sauce over the thighs, ensuring they're well-coated. Seal or cover and refrigerate for at least 1 hour.

Preheat grill to medium-high heat (400°F / 200°C).

Remove chicken from marinade and grill skin-side down for 6-7 minutes. Flip and grill for another 6-7 minutes until internal temperature reaches 165°F (75°C).

Transfer to a serving platter and drizzle with reserved chimichurri. Garnish with fresh herbs if desired.