01 - In a medium mixing bowl, combine the finely chopped parsley, cilantro (if using), minced garlic, olive oil, red wine vinegar, oregano, crushed red pepper flakes, black pepper, and kosher salt. Stir thoroughly until all ingredients are evenly incorporated. Reserve 1/3 cup of the chimichurri sauce in a separate container for finishing and set aside.
02 - Place the chicken breasts in a large resealable plastic bag or a shallow dish. Pour the remaining chimichurri sauce over the chicken, ensuring each piece is thoroughly coated. Seal the bag or cover the dish and refrigerate for a minimum of 1 hour, or up to overnight for optimal flavor penetration.
03 - Preheat an outdoor grill or grill pan to medium-high heat, approximately 400°F to 450°F. Remove the chicken from the marinade, allowing any excess to drip off. Discard the used marinade. Lightly brush the chicken with olive oil and season with salt and pepper to taste.
04 - Brush the grill grates with olive oil to prevent sticking. Place the chicken breasts on the grill and cook for 6 to 7 minutes per side, or until the internal temperature reaches 165°F when measured with a meat thermometer and the juices run clear.
05 - Transfer the grilled chicken to a serving platter and allow it to rest for 5 minutes to redistribute the juices. Slice the chicken diagonally and drizzle generously with the reserved chimichurri sauce before serving.