Using a peeler, peel 2 long ribbons of cucumber (if using). Gently fold back and forth, then secure on a skewer. Set aside for garnish.
In a cocktail shaker, gently muddle mint, elderflower liqueur, and lime juice until mint is wilted; do not break up the leaves.
Using a cocktail strainer or fine-mesh sieve, strain mint mixture into a blender. Add ice, Prosecco, and club soda and blend until combined.
Pour into wine glasses. Garnish with reserved cucumber (if using) and mint sprig.
