01 - Combine parsley, oregano, garlic, shallot, olive oil, vinegar, red pepper flakes, salt, and black pepper in a bowl. Mix thoroughly and set aside for 10 minutes to meld flavors.
02 - Measure out 1/3 cup of the chimichurri mixture and set aside separately for serving.
03 - Pat chicken thighs dry with paper towels. Season both sides evenly with salt and black pepper.
04 - Place chicken in a zip-top bag or shallow dish. Pour the remaining chimichurri over the meat, turning to coat. Marinate in the refrigerator for at least 30 minutes, up to 4 hours.
05 - Preheat grill to medium heat or oven to 400°F.
06 - Remove excess marinade to prevent burning. Grill skin-side down for 6-7 minutes per side until internal temperature reaches 165°F. Alternatively, bake on a parchment-lined sheet for 25-30 minutes.
07 - Let chicken rest for 5 minutes. Serve hot, drizzled with the reserved chimichurri.