Step 1

To make the rosemary simple syrup, bring the water, sugar and rosemary to a simmer in a small saucepan over medium heat. Simmer for 1 minute then remove from heat and allow to cool for at least 30 minutes. Strain.

Step 2

Add the ice to a large jug. Pour in the grapefruit juice, cooled syrup and sparkling water. Mix using a chopstick or the handle of a wooden spoon.

Step 3

Pour into glasses, adding grapefruit slices and rosemary stalks to serve, if desired.