Add vodka, grapefruit juice, lime juice, and agave nectar.
Muddle a rosemary sprig gently in the shaker.
Shake until chilled.
Strain into a glass packed with ice.
Top with sparkling water and stir gently.
Garnish with an extra rosemary sprig.
Notes
Use tart grapefruit juice for the brightest flavor; add extra agave nectar for a sweeter sip. Mint can substitute for rosemary if desired. Rim your glass with salt for a Paloma-style twist.