Add rosemary simple syrup and grapefruit juice. Fill the remainder of the glass with sparkling water (or Prosecco). Stir to combine.
Garnish with a spring of fresh rosemary and thin slice of grapefruit.
Notes
ROSEMARY SIMPLE SYRUP: you can find the recipe for this hereGRAPEFRUIT JUICE: I prefer to buy grapefruits and freshly squeeze them, but store bought grapefruit juice works too. Just look for unsweetened juice.MAKE IT A COCKTAIL: add Prosecco instead of sparkling water