Add the parsley and garlic in a food processor and pulse for a few minutes. You'll notice it's a little bit chopped use a spoon to scrape down the side as needed.
Add the red wine vinegar/apple cider vinegar, oil, and pulse further to combine. Scrape down the side if necessary.
Add red pepper flakes, salt, and pepper.
Add the chopped onion and pulse. For this step, you can just add the onion if you prefer that.
Taste and readjust if necessary. This sauce is good with chicken, beef/steak, garlic knot bread even vegetables. Store in an airtight or leave for about 15 minutes to allow the flavor to blend, serve and enjoy immediately. If you store in a refrigerator you will need to leave at room temperature before you can use it. Enjoy.