In a mixing bowl, combine chopped parsley, oregano, minced garlic, red wine vinegar, olive oil, salt, black pepper, and red pepper flakes to create the chimichurri sauce. Whisk together until well blended and slightly emulsified. Set aside to allow flavors to meld.
Pound the chicken to an even thickness or leave as is, then place it in a shallow dish or zip-top bag. Pour half of the chimichurri marinade over the chicken, ensuring all pieces are coated. Cover and refrigerate for at least 15 minutes, or up to 2 hours for more flavor.
Preheat the grill or grill pan over medium-high heat. Lightly brush with olive oil to prevent sticking.
Remove the chicken from the marinade, letting excess drip off. Place the chicken on the hot grill, and cook until golden brown and charred in spots—about 6-8 minutes per side, depending on thickness. You should hear a satisfying sizzle as it hits the hot grates.
Flip the chicken and continue cooking until the internal temperature reaches 165°F (75°C) and juices run clear. Remove from the grill and let rest for a few minutes to keep it juicy.
Slice the grilled chicken against the grain and transfer to a serving platter. Spoon additional chimichurri over the top for a vibrant, herby finish, and serve immediately.
