Boil water and sugar in a small pot.
Add rosemary sprigs and simmer for 5 minutes.
Remove from heat and let cool.
Strain and store in the fridge.
In a shaker, muddle the rosemary sprig gently.
Add grapefruit juice, lemon juice, and rosemary simple syrup.
Fill the shaker with ice and shake for 10-15 seconds.
Strain into a tall glass filled with ice.
Top with sparkling water and stir gently.
Garnish with a grapefruit slice and rosemary sprig.