Using 2 lbs sugar, scrape the zest from 3 lemons. Squeeze the juice [and set aside]. Add 1 small glass of water to the sugar and heat over a low fire, stirring until the sugar dissolves. Add the reserved lemon juice and, after putting more wood on the fire, boil over a high flame until the mixture sticks to the fingers and begins to thicken as for jelly. Remove the pan from the fire and stir in one direction with a wooden pestle, skimming off the foam and adding lemon juice for whiteness, just a few drops at a time. Keep stirring until the mixture thickens.
Transfer to a wide-mouthed jar and tie up. Eat with a spoon when drinking water or tea.