In a shaker, muddle the diced prickly pear with the mint leaves. (Muddling means gently pressing the ingredients with a bar spoon or muddler to release their flavors.)
If you don't have a bar spoon or muddler, you can use the back of a spoon to muddle the ingredients.
Add the tequila, lime juice, and grenadine syrup to the shaker.
You can adjust the amount of grenadine syrup to your taste preference.or a more intense prickly pear flavor, you can use prickly pear puree instead of diced prickly pear. To make the puree, simply blend the prickly pear flesh until smooth.
Fill the shaker with ice and shake vigorously for 15 seconds.
Strain the cocktail into a tall glass filled with fresh ice.
Top with soda water and garnish with a star fruit slice.