(1) Place the sugar, lemon juice, lemon zest and lime juice in a saucepan and boil for about 10 minutes.
(2) Pass the syrup through a seat and transfer to a pan. Freeze in the refrigerator for about 2-3 hours.
(3) When frozen, transfer the syrup to a blender with a little ice and the mint leaves. Beat well.
(4) Serve