Pour the lemon and mandarin juices into a saucepan, adding the castor sugar, and boil on a low heat until the sugar dissolves
Once completely cooled, pour a litre jug of ice cubes into a blender, and crush finely, then adding the juice mixture slowly until it becomes a creamy slush.
Add 1 to 2 tablespoons of Limoncello liquor to the mixture, for the alcohol version. Serve in individual liquor glasses, and garnish with a slice of mandarin dipped in Limoncello, along with a mint leaf, to the top of each glass.