Finely grate the zest from 4 lemons. Squeeze enough lemons to measure 150ml of fresh lemon juice.
Bring 1 liter of water and 150g of sugar to a boil. Simmer for 2 minutes, stirring until the sugar dissolves.
Add the lemon juice and zest. Remove from the heat, cover and cool for 20 minutes.
Strain into shallow freezer-safe containers and cool to room temperature. Freeze for several hours, until partly frozen.
Leave at room temperature just until the mixture can be broken into chunks. Pulse in a food processor until it resembles soft snow. Serve, or keep frozen until needed.
