This is best made in a food processor.
In the bowl of the processor, place the garlic and parsley leaves. Pulse on and off until they are minced fine. Scrape down the sides and add the vinegar, lemon juice and capers. Process to blend.
Now, with the processor running, slowly add the olive oil until well combined.
Taste and add a little salt if needed.
Feel free to adjust to your taste by adding a little more lemon juice or oil. You want it to be piquant but not too acidic.
This keeps well in the refrigerator, but bring it to room remperature before using.