How to Make It
In a cocktail shaker, muddle the grapes and 2 of the shiso leaves with the Simple Syrup. Add the verjus and ice cubes and shake well. Fine-strain into a chilled, crushed-ice-filled rocks glass. Stir in the club soda and garnish with the remaining shiso leaf.
Verjus, the tart juice pressed from unripe grapes, is available at specialty food stores.