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 prepared chimichurri sauce; this will be used for serving.
03 - Pat the chicken thighs dry using paper towels. Season both sides generously with the 1 teaspoon of salt and 1/2 teaspoon of 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 the meat is evenly coated. Marinate in the refrigerator for at least 30 minutes, up to 4 hours.
05 - Preheat the grill to medium heat or the oven to 400°F.
06 - Remove the chicken from the marinade and shake off excess liquid to prevent burning. Grill the chicken skin-side down for 6-7 minutes per side, or until the internal temperature reaches 165°F and the juices run clear. If baking, arrange on a parchment-lined baking sheet and cook for 25-30 minutes.
07 - Let the cooked chicken rest for 5 minutes before serving. Drizzle with the reserved chimichurri sauce.