Pat the chicken thighs dry with paper towels, then place them in a large bowl. Season with kosher salt and black pepper and set aside.

Prepare the chimichurri by combining parsley, garlic, olive oil, red wine vinegar, lemon juice, red onion, red pepper flakes, oregano, kosher salt, and black pepper in a food processor. Blend until slightly chunky but well combined.

Take half of the chimichurri sauce and pour it over the chicken thighs, making sure each piece is well coated. Cover the bowl and marinate for at least 20 minutes or up to overnight in the refrigerator. Reserve the remaining chimichurri for serving.

Preheat a cast iron grill pan over medium-high heat. Lightly oil the pan to prevent sticking.

Remove the chicken from the marinade and shake off any excess. Place the thighs on the hot pan and grill for 5-7 minutes per side, or until the internal temperature reaches 165°F (74°C). Avoid overcrowding the pan; cook in batches if necessary.

Transfer the cooked chicken to a plate and allow it to rest for 5 minutes before serving.

Serve the chicken warm, drizzled with the reserved chimichurri sauce on top. You can also serve additional chimichurri on the side. Enjoy!