1 Put the sugar in a pan together with 150 ml water. Heat until all the sugar has dissolved, then simmer for a few minutes until the liquid resembles a thick syrup. Remove from the heat and stir in the freshly squeezed lemon and the Limoncello.
2 Chill in the fridge for 30 minutes. Whisk the egg whites until stiff, then fold under the lemon syrup solution. Pour into a flat container. Freeze for 3 hours, stirring every 30 minutes with a fork to break up the ice.
3 Cut the lids off the lemons and scrape out the lemon from the inside (use otherwise). Wash out using hot water and place in the freezer.
4 Scoop some of the sorbet in the lemons and serve.