01 - In a large bowl, combine the parsley, oregano, garlic, shallot, olive oil, vinegar, red pepper flakes, salt, and black pepper. Mix thoroughly and set aside for at least 10 minutes to allow the flavors to meld.
02 - Measure out and reserve 1/3 cup of the prepared chimichurri sauce specifically for serving later.
03 - Use paper towels to pat the chicken thighs completely dry. Season both sides generously with the 1 teaspoon of salt and 1/2 teaspoon of black pepper.
04 - Place the seasoned chicken thighs into a zip-top bag or a shallow dish. Pour the remaining chimichurri sauce over the chicken, ensuring pieces are turned to coat evenly. Marinate in the refrigerator for at least 30 minutes, or up to 4 hours for deeper flavor.
05 - Preheat the grill to medium heat or the oven to 400°F (200°C).
06 - Remove the chicken from the marinade, shaking off excess liquid to prevent burning. Grill the thighs skin-side down for 6-7 minutes per side, or until the internal temperature reaches 165°F (74°C). If baking, arrange on a parchment-lined sheet and cook for 25-30 minutes.
07 - Allow the cooked chicken to rest for 5 minutes before serving. Drizzle with the reserved chimichurri sauce.