Measure out elderflower syrup and lime juice and add them to a white wine glass. Add mint leaves.
Muddle the leaves slightly (bruise them) to infuse the cocktail with the mint flavor. You can do this with a muddler or just with a handle of a wooden spoon.
Add a couple of ice cubes.
Fill the glass with prosecco and soda water.
Stir gently and adjust the drink to your liking by adding more lime juice or syrup if necessary.
Enjoy!