Bring the sugar, water, and lemon zest to the boil, then let it cool completely.
Add the lemon and orange juice, and, if you like, a few drops of red food colouring.
Strain into a shallow dish and freeze into a sorbet, raking thoroughly with a fork every 30-40 minutes (or using an ice cream maker), over about 3 hours.
Pack the frozen sorbet into a mould, and keep solidly frozen until serving.
Just before serving, whisk the egg whites with the sugar into stiff peaks.
Turn the sorbet out onto a heatproof plate, and quickly cover it completely with the meringue.
Brown the surface of the meringue for a few seconds under a very hot oven grill, or with a kitchen torch, taking care that the sorbet inside doesn't melt.
Bring it to the table at once, top with a sugar cube soaked in rum, and carefully light it, keeping well away from any open flame.
Based on: Malatinszky Fanny szakácskönyve (a classic Hungarian cookbook), modernised. Calories and protein are estimated.