01 - In a bowl, combine parsley, oregano, garlic, shallot, olive oil, vinegar, red pepper flakes, salt, and black pepper. Mix well, then let sit for at least 10 minutes for flavors to meld.
02 - Reserve 1/3 cup of the chimichurri for serving.
03 - Pat the chicken thighs dry with paper towels. Season with salt and black pepper on both sides.
04 - Place chicken thighs in a zip-top bag or shallow dish. Pour 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.
06 - Remove excess marinade from the chicken to avoid burning. Grill chicken thighs skin-side down over medium heat for 6-7 minutes per side, or until cooked through and juices run clear (internal temp 165°F). Alternatively, bake on a parchment-lined sheet for 25-30 minutes.
07 - Let rest for 5 minutes. Serve hot, drizzled with reserved chimichurri.