In a mixing bowl, combine the parsley, cilantro, garlic, red pepper flakes, salt, and black pepper.

Whisk in the olive oil and red wine vinegar until well combined.

Set aside to let the flavors meld while preparing the chicken.

Place the chicken thighs in a large bowl or zip-top bag.

Pour half of the chimichurri sauce over the chicken, reserving the other half for serving.

Marinate in the refrigerator for at least 30 minutes, or up to 2 hours for more flavor.

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 from the grill and let rest for 5 minutes before serving.

Serve the grilled chicken thighs with the reserved chimichurri sauce drizzled on top.