Wash the lemon balm and pat dry. Separate the leaves from the stems. Bring the water to boil in a saucepan. Add the sugar, citric acid and the yellow lemon peel, thinly peeled with a sharp knife (be careful, the white skin underneath is bitter). Boil for a few minutes and then leave to cool.
Pour the lemon juice into the sugar water. Add the lemon balm leaves. Cover with cling film and allow to rest for five days in a cool, dark place (in the cellar). Stir occasionally.
Strain syrup through a fine cloth into a clean pot, then pour into sterilised bottles. Cover the bottles with a screw cap only, but do not screw them shut. The syrup may occasionally ferment and the bottles could crack. Store the bottles in a fresh and cool place. lemon balm syrup keeps well for six months.
Mix the syrup with soda water (ratio 1:6) and a few slices of lemon or the juice of a fresh lemon. For a Hugo, replace most of the water with Prosecco and add a few fresh mint or lemon balm leaves.
Notes
The more leaves you use, the more intense the aroma will be.