In a medium saucepan, dissolve the sugar in the water and bring to a boil, stirring. Cook, uncovered, without stirring, for 5 minutes. Pour this into a ceramic bowl and refrigerate for at least 2 hours. Add the juice and zest and stir. Pour into an ice cream maker and follow the manufacturer’s instructions or, if you don’t have an ice cream maker, simply freeze in a metal pan for 5 to 7 hours. Just before serving, break into pieces and pulse quickly in a blender or food processor. Add garnish.