Add the cucumber slices and fresh lemon juice to a cocktail shaker.
Muddle well until the cucumber releases its juices.
Add the vodka, elderflower liqueur, simple syrup, and a generous handful of ice.
Shake vigorously for about 20 seconds until the shaker is well chilled.
Fill a martini glass with fresh ice.
Strain the cocktail over the ice and garnish with a cucumber ribbon or lemon twist, if desired.
Serve immediately and enjoy!