Put the sugar in a pan and pour in 300 ml cups water. Bring to the boil, stirring occasionally until the sugar has just dissolved.
Using a vegetable peeler (I did not have it so I used a fine grater), pare the rind thinly from two of the lemons and put in a pan. Simmer for 2 minutes, then take the pan of the heat. Leave to cool then chill.
Squeeze all the juice from the four lemons and add it to the syrup. Strain the syrup into a shallow freezerproof container, reserving the rind. Freeze for 4 hours, until it is mushy.
Process the sorbet in a food processor until it is smooth. Lightly whisk the egg white with a fork until it is just frosty (used an electric mixer). Replace the sorbet in the container, beat in the egg white and return to the freezer for 4 hours, or until it is firm.
Cut the reserved lemon rind into fine shreds and cook in boiling water for 5 minutes. Drain, then place on a plate and sprinkle with sugar. Scoop the sorbet into bowls or glasses and decorate with the sugared lemon rind.
