Step 1Heat the water and sugar just until clear, about 5 minutes. Add the lemon zest and remove from heat. Let the syrup steep for at least 30 minutes.
Step 2Stir in the lemon juice. Strain the mixture into a shallow 9-by-12-inch glass baking dish.
Step 3Freeze the mixture for an hour. Remove from the freezer and stir with a fork, breaking up any chunks of ice. Return it to the freezer. Repeat 4 or 5 times over the next 2 to 3 hours. Each time, the ice will be a little less liquid and will stick together more. When it is firm enough to hold a shape, it is done.
Step 4Try not to let the ice freeze solid. If it does, chop it into small pieces in the dish and grind it in the food processor. (The result will be lighter and fluffier and the flavor will not be as intense.)