01 - In a large bowl, combine the 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 specifically for serving.
03 - Pat the chicken thighs completely dry using paper towels. Generously season both sides of the thighs with 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 marinade over the chicken, ensuring each piece is evenly coated. Seal the container and marinate in the refrigerator for at least 30 minutes, up to 4 hours.
05 - Preheat your grill to medium heat or your oven to 400°F.
06 - Remove the chicken from the marinade and shake off any excess liquid to prevent burning. Grill the thighs skin-side down over medium heat for 6 to 7 minutes per side, or until the internal temperature reaches 165°F and juices run clear. Alternatively, bake on a parchment-lined baking sheet for 25 to 30 minutes.
07 - Transfer the cooked chicken to a cutting board and let it rest for 5 minutes. Serve hot, drizzled with the reserved chimichurri sauce.