Celery juice is a crucial element of this cocktail; if you don't own a juicer, blend the celery in a high-powered blender, then strain it through cheesecloth, bundling the solids together to extract as much juice as possible. Alternatively, if you live near a juice bar, ask to buy a few ounces of celery juice there.
1-1/2 ounces dry gin, such as Junipero or Hendrick's
1/2 ounce freshly squeezed lime juice
1/2 ounce simple syrup
3/4 ounce fresh celery juice (from about 1 celery stalk)
7 or 8 fresh mint leaves, plus more mint for garnish
Chill a champagne coupe or other stemmed glassware by placing in the freezer or by filling the glass with ice while the cocktail is constructed.
Add all ingredients, except the reserved mint, to a cocktail shaker. Fill with ice to the brim, and chill by gently inverting the cocktail shaker about 10 times.
Strain into the chilled glass, and garnish with the small tender leaves of the mint plant, gently massaging the mint leaves between your fingertips to release their oils first.