Prepare agave syrup by diluting agave nectar with water in 1 : 1 ratio.
In a Boston shaker, lightly muddle basil leaves, agave syrup and freshly squeezed lime juice. Add Arrogante Reposado tequila, Aperol, and freshly squeezed juice of ruby red grapefruit. Stir well. Add a handful of ice and shake vigorously for 10 to 15 seconds.
Place a large ice ball or a large hand cut ice cube in a highball glass. Strain the cocktail through a fine mesh strainer into the prepared glass. Add a splash of the chilled club soda to finish the cocktail and gently stir to combine.
Garnish the cocktail with a basil leaf or a wedge of red grapefruit.
Michael Glassberg (Carousel Bar, New Orleans)