Place 3 cucumber slices in the bottom of a cocktail shaker.
Muddle the cucumber gently to release the juice and oils.
Add gin, elderflower liqueur, and fresh lime juice to the shaker.
Fill the shaker with ice cubes.
Shake vigorously for 15 seconds until well-chilled.
Double-strain the mixture through a fine-mesh strainer into a chilled cocktail glass.
Garnish with a fresh cucumber slice or ribbon.
Notes
London Dry gin works best because its strong juniper notes pair well with cucumber and elderflower.Use fresh lime juice for the best taste.Store St. Germain elderflower liqueur in the refrigerator after opening. This keeps its flavor fresh.For a non-alcoholic version, replace gin and elderflower liqueur with cucumber-infused water and elderflower syrup.