Blend the watermelon with a food processor or an immersion blender. Add the lime juice, the lime from which you squeezed the juice and the star anise.
Let it rest for at least 1 hour.
For every drink, pour ¼ cup + 1 tbsp (75ml) of watermelon juice, ¼ cup (60ml of tonic water and, if you want an alcoholic drink, 1 tbsp (15ml) of vermouth. Serve with lime slices and ice.
Recipe by The Adagio Blog at https://adagioblog.com/watermelon-tonic/