Prepare the cucumber: Using a vegetable peeler, create long ribbons from half the cucumber. Set these aside for garnish. Roughly chop the remaining cucumber into chunks.
Muddle the cucumber: Place the cucumber chunks in a cocktail shaker or mason jar. Using a muddler or the back of a wooden spoon, gently crush the cucumber to release its refreshing juices and flavor.
Add the liquid ingredients: Pour the elderflower syrup and lime juice into the shaker with the muddled cucumber. If using additional sweetener, add it now.
Add ice and shake vigorously: Add about a cup of ice to the shaker and shake for 15-20 seconds until the outside of the shaker becomes frosty and cold.
Strain into glasses: Fill four tall glasses with fresh ice. Strain the mixture evenly into each glass, filling them about halfway.
Top with sparkling water: Slowly pour sparkling water into each glass, leaving about half an inch of room at the top. The ratio should be roughly 1 part cucumber-elderflower mixture to 1 part sparkling water.
Garnish beautifully: Gently drape the reserved cucumber ribbons around the inside of each glass. Add a cucumber slice to the rim and tuck a sprig of fresh mint into each drink.
Serve immediately: Give each drink a gentle stir before serving to combine the flavors while maintaining the effervescence.