Prepare the Rosemary Syrup. In a small saucepan over medium heat simmer the water and sugar until the sugar has melted. Add the rosemary, reduce heat and gently simmer for 8 minutes. Turn the heat off, allow to cool. Strain the rosemary.
Store in the refrigerator for up to 2 weeks.
Combine the grapefruit, syrup, vodka and bitters in a shaker filled with ice. Cover and shake, pour over ice and garnish with a sprig of rosemary.
Recipe by phoebe's pure food at https://www.phoebespurefood.com/rosemary-greyhound/