First prepare the rosemary infused sugar syrup by boiling equal parts water and sugar. When the sugar has melted, take the syrup off the heat, put in the rosemary and lemon zest and let it cool down. You can store this syrup in your refrigerator for a few weeks.
To make the cocktail, put the gin, freshly squeezed lemon juice and sugar syrup in a glass with ice cubes. Pour in sparkling water.
Garnish with rosemary and lemon. Serve and enjoy!