01 - Combine parsley, oregano, garlic, shallot, olive oil, vinegar, red pepper flakes, salt, and black pepper in a bowl. Mix well and let sit for at least 10 minutes to meld flavors.
02 - Set aside 1/3 cup of the chimichurri for serving.
03 - Pat chicken thighs dry with paper towels. Season both sides with salt and black pepper.
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 grill or oven to 400°F.
06 - Remove excess marinade from chicken to prevent burning. Grill skin-side down over medium heat for 6-7 minutes per side until cooked through (internal temp 165°F). Alternatively, bake on a parchment-lined sheet for 25-30 minutes.
07 - Let chicken rest for 5 minutes. Serve hot, drizzled with reserved chimichurri.