Optional: Rim the glass with salt or sugar by wetting the rim and dipping it into a shallow plate.
Fill a cocktail shaker with ice and add tequila, grapefruit juice, lime juice, and lavender syrup.
Shake well for about 10–15 seconds.
Strain into a glass filled with fresh ice.
Top with sparkling water to your preference.
Garnish with a sprig of lavender and a grapefruit slice.