Finely chop cilantro, onion, and garlic and mix in a small bowl.
Add the oil, vinegar, and salt and mix well.
If you keep this in the refrigerator, bring back to room temperature before serving.
Notes
These measurements are for the chimichurri to be served on the side as a condiment. If you're using it as a marinade, it's very easy to double, or triple the size to accommodate for the amount of meat. For each pound of meat, double the recipe.