Pulse the garlic and onion in the food processor until finely chopped.
Add the parsley and cilantro and pulse briefly, until finely chopped.
Transfer the mixture to a separate bowl. Add the olive oil, lime juice, and vinegar, and stir. (Adding the liquids outside of the blender gives the chimichurri the correct texture. You don't want the herbs to be completely puréed, just finely chopped).