Step 1 - Scrub the lemons and then strip the zest from them and squeeze the juice out.
Step 2 - Put the lemon juice and zest, sugar and tonic water into a heavy based saucepan and heat gently. Stir occasionally until the sugar has dissolved and has become syrupy. Remove from the heat and leave to cool.
Step 3 - Put the sorbet mix into a jug and cool in the fridge for at least an hour.
Step 4 - Add the beaten egg white and gin into the the jug and mix.
Step 5 - Transfer the mix to a tuperware tub and put into the freezer. After a couple of hours open the tub and fork up the mixture. Do this again two hours later.