To make the syrup, add the water, sweetener, rosemary, ginger, and black pepper to a saucepan. Bring to the boil, and after one minute remove from the heat and let stand for one hour.

Strain the syrup through a mesh sieve, pushing down on the solids to extract as much of the liquid as possible. It will make about a half-cup of syrup, which you can make up to a day in advance.

When you're ready to enjoy the cocktail, add the syrup to a cocktail shaker together with the gin, lime juice, and a handful of ice cubes.

Shake well, strain into two glasses filled with fresh ice, then add a splash of club soda.

Garnish with lime slices and rosemary stalks.