Muddle the strawberries in a cocktail shaker. Add the tequila, fresh lime juice, fresh grapefruit juice, and simple syrup. Fill the shaker with ice.
Shake the mixture vigorously for about 15 seconds.
Strain the cocktail mixture into a glass that's filled with ice. If you fancy a salty touch, rim the glass with salt before pouring. Top it off with a splash of club soda.