Place lime, cucumber and sugar syrup into glass part of Boston shaker and muddle with muddler for about 1 minute. Add mint leaves and vodka.
Prepare glass by sticking cucumber on inside of glass. Fill glass with crushed ice.
Place ice cubes into Boston shaker, place metal tin part on top, then shake vigorously for about 30 seconds. Using hawthorn strainer strain cocktail from metal tin into the prepared glass. Top with sparkling/soda water and garnish with mint sprig.