Bring 450 ml of water and sugar to a short boil while stirring. Let cool off.
Squeeze the lemons and measure 100 ml juice. Mix sugar syrup, prosecco and lemon juice in a bowl. Place in the freezer. Freeze for at least 2 hours.
Beat egg whites until stiff. Stir the frozen mixture vigorously. Fold in the beaten egg white. Let everything freeze for another 3 hours, stirring every 30 minutes.
Select the raspberries, puree them with icing sugar and pass them through a sieve. Cut out balls from half of the sorbet and arrange in four pre-cooled bowls. Refreeze the rest of the sorbet. Decorate with raspberry puree, lemon balm and biscuits.