Cut ginger in thin slices and vigorously roll rosemary between your hands. Place them in a small saucepan and add 1 cup of water. Cover and bring to a boil, then remove from heat and leave to steep for 20 minutes.
Pour grapefruit and lime juices in 4 glasses. Add ice, almost to the top of the glass. Pour over the ginger and rosemary syrup, and white rum.
The recipe for the syrup is for 4 glasses. If you are making a batch, you can multiply it.
Mocktail version: Omit the white rum or replace it with sparkling water.
Recipe by Anja Burgar | Use your Noodles | www.useyournoodles.eu