In a cocktail shaker, add the 3 thin cucumber slices and 6 mint leaves. Add the simple syrup (0.5 oz). Gently muddle 6–8 times just to release cucumber juice and mint aroma — avoid pulverizing the mint into bits.

Add spirits and citrus

Add 2 oz white rum, 0.5 oz elderflower liqueur, and 0.75 oz fresh lime juice to the shaker.

Shake with ice

Fill the shaker with ice (about 1 cup cubed). Secure the lid and shake vigorously for 10–12 seconds until the outside of the shaker becomes cold and slightly frosted.

Strain

Double-strain the cocktail into a chilled highball or Collins glass filled halfway with fresh ice (use a fine mesh strainer plus the shaker strainer) to remove mint bits and cucumber solids.

Top and stir

Top with 2 oz club soda. Gently stir once or twice with a bar spoon to integrate soda without losing carbonation.

Garnish and serve

Garnish with a lime wheel on the rim and drape a long cucumber ribbon inside the glass. Optionally slap a small mint sprig between your palms and float it on top for aroma. Serve immediately.