Add the parsley, cilantro, red wine vinegar, chopped onion, garlic, salt, oregano, hot pepper flakes, and black pepper to the bowl of a food processor.
Pulse a few times until everything is finely chopped but still textured—don’t let it turn into a paste.
With the processor running, slowly drizzle in the olive oil. Pulse just until the sauce comes together and looks loose, fresh, and spoonable.
Transfer the chimichurri to a container and refrigerate for about 2 hours so the flavors have time to blend. Stir before serving.