Mix the water and sugar in a saucepan. Bring to a boil over a high flame, stirring until the sugar is completely dissolved. Lower the flame and simmer for five minutes, or until the mixture is syrupy and clear. Turn off the heat and let the syrup cool.

In a bowl, mix together the plum puree and sugar syrup. Add the lemon juice. Pour this mixture into a loaf pan. Place the pan in the freezer.

After 30–60 minutes, take a fork and fluff up the ice crystals that have formed on the top of the mixture. Return the pan to the freezer and repeat this every 30 minutes or so. It should take upwards of five or six hours for the granita to be fully ready, with no liquid on the bottom. Spoon the granita into serving glasses.