In a food processor, pulse the parsley, cilantro, garlic, oregano, red pepper flakes, salt, and black pepper until finely chopped.

Transfer the chopped herb mixture to a bowl. Stir in the olive oil and red wine vinegar until well combined. Reserve exactly half of this sauce in a separate bowl for serving.

Place the meat in a large bowl or zip-top bag and pour the remaining half of the sauce over it. Toss to coat evenly. Refrigerate and marinate for at least 30 minutes.

Preheat a grill or large skillet over medium-high heat. Cook the meat for 6 to 8 minutes per side, or until the internal temperature reaches 165°F on an instant-read thermometer.

Transfer the meat to a cutting board and let it rest for 5 minutes. Serve warm, topped with the reserved sauce.