2 oz. blanco Tequila
1 oz. fresh grapefruit juice
3/4 oz. fresh lime juice
1/2 oz. Agave nectar or simple syrup
2 dashes orange bitters
Splash of sparkling water
Garnish with a half moon grapefruit slice
Combine all the ingredients except the sparkling water into a shaker. Add ice, shake vigorously, and strain into a collins glass. Add fresh ice and top off with sparkling water and garnish.