Combine the sugar, water, and 2 rosemary sprigs in a small saucepan over medium heat; stir constantly for 2 minutes, until the sugar dissolves completely and the syrup looks clear.

Remove the pan from the heat and let the syrup steep for 10 minutes so the rosemary perfumes it, then strain out the sprigs and cool the syrup to room temperature; it keeps covered in the fridge for 2 weeks.

Fill a large wine glass with ice cubes until they reach the rim.

Pour 1 ounce of the cooled rosemary syrup and the grapefruit juice over the ice.

Top with the chilled tonic water and stir once with a long spoon, just enough to combine without losing the fizz.

Garnish with the grapefruit wheel and the extra rosemary sprig, clapping the sprig between your palms first to release its oil before dropping it in.