If you enjoy a salted rim, prep the rim of your glass by rubbing a lime wedge along the rim. Dip in tequila salt, and set aside.
Make the cocktail:
Fill a cocktail shaker with ice and add grapefruit juice, tequila, lime juice, and simple syrup (if using). Shake well until the sides are fully frosted.
Serve:
Fill your glass with fresh ice, pour the cocktail through the strainer into your glass, and top the drink with sparkling water.
Garnish with a lime or grapefruit wedge and a rosemary sprig -if you want to be extra.