Step 1

In a small food processor, combine the parsley and garlic. Process until finely chopped, stopping to scrape down the sides as necessary.

Step 2

Add the olive oil, vinegar, salt and red pepper flakes to the food processor (don’t add the onion just yet). Process briefly to combine.

Step 3

Transfer the mixture to a small jar or bowl and whisk in the red onion until the mixture is well blended. Let the sauce rest for 10 minutes to allow the flavors to meld. Taste, and if necessary, add additional salt (sometimes I add another 1/4 teaspoon) and/or red pepper flakes (for more heat).

Step 4

The sauce tastes best when served fresh, but it keeps well, covered and refrigerated, for 3 to 4 days. To prevent the sauce from turning a darker color (which doesn’t affect flavor), press plastic wrap directly against the surface before storing. If the olive oil solidifies in the refrigerator, don’t worry—let it warm up for 5 to 10 minutes at room temperature and then whisk to recombine.