The juniper flavor that predominates in gin goes really well with the herbs and celery in this refreshing combo. To extract the most vivid color from the herb leaves, don't hold back when you muddle (bruise) them.
Using a wooden spoon or muddler, vigorously bruise parsley and mint leaves with gin in a cocktail shaker (or use a mortar and pestle, then scrape into shaker). Add simple syrup and strain into 2 small ice-filled glasses. Pour 1/2 cup club soda into each and stir. Garnish glasses with celery stalks and mint sprigs.