1½ oz tequila1 oz red grapefruit juice¾ oz lime juice½ oz agave syrup2 dashes grapefruit bitterssoda water
instructions :

Add all the ingredients except soda water to a cocktail shaker. Add ice and shake until chilled.

Strain into a Collins glass over ice and top with soda water.

Garnish with a half wheel of grapefruit.

source :