Place the parsley leaves on a cutting board, and grate the garlic over the leaves. Finely chop the two together and transfer to a small bowl. Zest the lemon over the mixture, then rub together with your fingers to combine and release the citrus oils (the mixture will start to look shiny and moist). Add more zest (or a pinch of salt) and rub with your fingers as needed to moisten the mixture. Store in the refrigerator for up to one day.