01 - In a mixing bowl, combine the 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 well blended.
02 - Set aside ⅓ cup of chimichurri for finishing. Place the chicken breasts in a large resealable bag or shallow dish, pour the remaining chimichurri and olive oil over them, and toss to coat evenly. Refrigerate for at least 1 hour or up to 8 hours for deeper flavor.
03 - Heat a grill or grill pan to medium-high. Remove the chicken from the marinade and discard the used marinade.
04 - Cook the chicken 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.
06 - Arrange the sliced chicken on a platter and drizzle with the reserved chimichurri.