Place cucumber slices in the cup of a cocktail shaker along with fresh lime juice, then muddle to release the juices.
Fill the cocktail shaker halfway with ice, then add gin, dry Vermouth, and St Germain. Shake well to aerate or stir.
Place the remaining cucumber slices or rounds in a chilled martini glass, then strain the drink into the cup.
It’s light, crisp, and drinks easily (the best cocktails do!).