Cut the lime and run it around the rim of a chilled highball glass. Put some salt in a small dish and upturn the glass to coat the rim in the salt.
Put the tequila, grapefruit juice, 1 tbsp of juice from the lime and the agave syrup in a cocktail shaker with ice and shake well.
Strain into the glass, top up with sparkling water and garnish with a wedge of grapefruit.
