Run a lime wedge around the rim of a highball glass and dip into coarse salt to rim, then fill the glass with ice cubes.

In a cocktail shaker, gently muddle one sprig of rosemary with the agave nectar to release the oils, being careful not to over-muddle.

Add tequila, grapefruit juice, lime juice, and ice to the shaker, then shake well for 10-15 seconds.

Strain into the prepared glass, top with sparkling water, and garnish with a grapefruit slice and the remaining rosemary sprig.