To make the Rosemary Simple Syrup, add the water, sugar, and rosemary to a saucepan.

Cook over medium heat until the sugar dissolves.

Remove from heat and let the rosemary steep in the syrup. Strain the rosemary and pour it into a container. Let cool completely.

Add the grapefruit juice, vodka, rosemary syrup, and lemon into a cocktail shaker with ice.

Shake and pour over ice into 2 glasses. Garnish with rosemary sprig and grapefruit slice.