Make cucumber juice. Grate ½ cucumber and squeeze through a muslin cloth or press in a sieve. You’ll get roughly 80 ml from a medium cucumber. Keep the juice in the fridge if you make it ahead (up to 2 days).
Chill glasses. Place two martini glasses in the freezer while you prep. Cold glass = crisp experience.
Shake it up. Add gin, cucumber juice, St Germain, lime juice, and sugar syrup into a shaker half filled with ice. Shake hard for 30 seconds.
Pour & garnish. Divide the drink between the two chilled glasses. Use a vegetable peeler to make long thin cucumber ribbons and pleat them onto a cocktail stick for a pretty garnish.
Bold serving note: Serve immediately. The cucumber flavor tastes freshest right after shaking.