- Make rosemary simple syrup by combining sugar, water and chopped rosemary in a small saucepan over medium heat. Simmer until sugar dissolves (about 3 minutes) and strain. Cool completely and store in a resealable container.
- Pour grapefruit juice and 2 Tbsp rosemary simple syrup into a glass and stir.
- Fill glass with crushed ice, then top with seltzer water.
- Garnish with rosemary sprig to serve.