01 - In a large bowl, combine parsley, oregano, garlic, shallot, olive oil, vinegar, red pepper flakes, salt, and black pepper. Mix thoroughly and allow the mixture to sit for at least 10 minutes to let the flavors meld.
02 - Measure out and set aside 1/3 cup of the chimichurri sauce in a separate small bowl for serving later.
03 - Pat the chicken thighs dry with paper towels. Season both sides generously with the measured salt and black pepper.
04 - Place the seasoned chicken thighs in a zip-top bag or a shallow dish. Pour the remaining chimichurri sauce over the chicken, ensuring each piece is evenly coated. 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 excess marinade from the chicken to prevent burning. Grill the thighs skin-side down for 6-7 minutes per side until the internal temperature reaches 165°F (74°C) and juices run clear. Alternatively, bake on a parchment-lined baking sheet for 25-30 minutes.
07 - Let the cooked chicken rest for 5 minutes to retain juices. Serve hot, drizzled with the reserved chimichurri sauce.