Combine all the ingredients except the sparkling water in a cocktail shaker filled with ice and shake.
Pour everything over ice in a long glass frosted with sugar and salt.
Add sparkling water and garnish with a grapefruit slice.
45 ml Patrón Reposado
82.5 ml Fresh grapefruit juice
15 ml Agave syrup (1: 1)
15 ml Freshly squeezed lime juice
30 ml Sparkling water
+ Sugar and salt icing
+ Grapefruit slice to garnish
Alcohol 40 %