Grate the cucumber using a box grater or in a food processor. Use your hands or a muslin cloth to extract the juice then strain into a clean jar and keep in the fridge for up to 2 days. You will get rougly a half a cup of juice from 1 medium cucumber.
Place two martini glasses in the freezer to chill.
Put the gin, cucumber juice, St Germain, lime juice, and sugar syrup in a cocktail shaker half filled with ice.
Shake hard for 30 seconds and divide between the two chilled glasses.
Use a vegetable peeler to peel long thin slices from a cucumber and the pleat them onto a cocktail stick to garnish.
If you haven't got any St Germain you could replace with some eldreflower cordial.