In a small saucepan, heat 250ml water, the sugar, and the lemon peel until the sugar dissolves, then bring to a boil. Cook for 3 minutes, then remove from the heat and set aside to cool. Remove and discard the lemon peel. 100ml lemon juice, along with the vodka if using, should be added to the sugar mixture.

Pour into a freezer box and freeze for 1 hour and 30 minutes, then whisk to break up and incorporate the ice crystals (which will develop around the edges) before returning to the freezer.

For 4 hours, stir the sorbet once an hour to break up the ice crystals. When the mixture is hard but still scoopable, stop mixing and place in the freezer for up to 1 month. Serve sorbet scoops garnished with lemon zest curls.