01 - In a mixing bowl, combine the finely chopped parsley, cilantro (if using), minced garlic, red wine vinegar, extra-virgin olive oil, dried oregano, red pepper flakes, kosher salt, black pepper, and lemon juice. Stir until all ingredients are evenly incorporated.
02 - Reserve 1/3 cup of the chimichurri for serving. Place the chicken breasts in a large resealable bag or shallow dish, then add the remaining chimichurri and olive oil. Toss to coat evenly and refrigerate for at least 1 hour, or up to 8 hours for deeper flavor penetration.
03 - Preheat your grill or grill pan to medium-high heat. Remove the chicken from the marinade, allowing excess to drip off, and discard the used marinade.
04 - Place the chicken on the hot grill and cook for 6 to 8 minutes per side, until the internal temperature reaches 165°F and the juices run clear.
05 - Transfer the grilled chicken to a cutting board and let it rest for 5 minutes before slicing to allow the juices to redistribute.
06 - Slice the chicken and arrange on a serving platter. Drizzle the reserved chimichurri generously over the top and serve immediately.