In a medium bowl, combine chopped flat-leaved parsley, minced garlic, extra virgin olive oil, red wine vinegar, dried oregano, fine sea salt, freshly ground black pepper, and crushed red pepper flakes. Stir well and let sit in the fridge for at least 2 hours or overnight.

Place the chicken thighs in a large bowl and sprinkle with fine sea salt. Pour half of the chimichurri sauce over the chicken and marinate at room temperature for 15 minutes or refrigerate overnight for more flavor.

Preheat the grill to medium-high heat (375°F to 450°F). Grill the marinated chicken thighs smooth-side down for 5-7 minutes without moving them. Flip and cook for another 5-7 minutes until the internal temperature reaches 165°F.

Remove the chicken from the grill and let it rest for a few minutes. Plate the grilled chicken and spoon the remaining chimichurri sauce on top.