Put the water and stevia in a saucepan over a low heat. Stir the mixture well. When it starts to boil, remove from the heat.
Leave the mixture to rest for 5 minutes.
Place the lemon juice, zest and water/sweetener mixture in a bowl.
Combine all the ingredients well.
Leave in the freezer for at least 3 hours.
To serve the sorbet, take out of the freezer, put it in the microwave for 15 seconds and leave it to stand for another 1 minute. This will give it the normal consistency of sorbet.