01 - In a bowl, combine parsley, oregano, garlic, shallot, olive oil, vinegar, red pepper flakes, salt, and black pepper. Mix well and let sit for at least 10 minutes to allow flavors to meld.
02 - Set aside 1/3 cup of the prepared chimichurri sauce for serving later.
03 - Pat the chicken thighs dry with paper towels and season with salt and black pepper on both sides.
04 - Place chicken thighs in a zip-top bag or shallow dish. Pour the remaining chimichurri over the chicken, turning to coat evenly. Marinate for at least 30 minutes, or up to 4 hours in the refrigerator.
05 - Preheat the grill or oven to 400°F (200°C).
06 - Remove excess marinade from the chicken. Grill skin-side down over medium heat for 6-7 minutes per side, or until cooked through and juices run clear (internal temperature 165°F/74°C). Alternatively, bake on a parchment-lined sheet for 25-30 minutes.
07 - Let the chicken rest for 5 minutes. Serve hot, drizzled with the reserved chimichurri.