Juice all the fruits and keep the empty shells to serve the granita.

Tip the juice, zest, lemongrass, lime leaf (if using), and sugar into a bowl.

Pour over 500ml boiling water.

Stir everything together until the sugar has dissolved, then leave to cool and infuse.

Strain the lemon syrup into a tray and freeze as stated.

Place the lemon and lime halves in the freezer and fill with granita to serve