John uses a mandolin to make his ultra-thin slices of fennel. Any hand-held slicer will do the trick.
Muddle the fennel slices and basil at the bottom of a cocktail shaker. Add gin, St. Germain, lime juice, and, if using, fennel bitters; shake well with ice, and double-strain into an ice-filled rocks glass. Garnish with the basil sprig, fennel slice and fennel frond.