Pour the tequila, agave, lime juice and grapefruit juice into a cocktail shaker. Add a large handful of ice and shake until the shaker feels really cold.
Rub the grapefruit wedges around the rim of 2 tall glasses then dip into the salt so it sticks.
Fill your prepared glasses with ice cubes then pour over the cocktail and divide the sparkling water between them. Stir gently to combine and garnish with the grapefruit wedges and a sprig of rosemary, if you like.
