2) Using the finest grater you have, lightly scrape the skin of one lemon, removing only the top surface of the peel. Squeeze out the juice and remove the seeds from both lemons.
3) Open the champagne or sparkling wine and pour it into a 23 by 30 by 5-cm pan. Mix in the simple syrup, lemon juice and grated zest. Place the mixture into the freezer.
4) Check the mixture after 30 minutes. Using a wooden spoon, break up any ice formed on the sides and stir it into the rest of the liquid. Continue to freeze and repeat this procedure every hour for at least 3 hours. Before serving, fluff the granita by chopping up any clumps with the spoon or gently whisking the frozen mixture. Spoon into small glasses or cups and serve.