Peel the nectarines, remove the stone and chop them in a bowl.
Pour into a saucepan water and sugar, add the sprigs of lemon balm and bring to a gentle boil until the sugar dissolves and becomes a syrup.
Remove the sprigs of lemon balm and pour the sugar syrup in a blender with the chopped nectarines, then add the prosecco. Blend until smooth, then stash it in the fridge for a few hours until completely cold.
When it is ready, pour it into the ice cream maker and follow maker's directions.
When the sorbet has the right texture, serve it immediately or transfer it in a Tupperware and place it in the freezer until ready to serve.