Fill each glass with a handful of crushed ice and some torn mint leaves to taste (3 or 4).
Muddle the mint leaves to release the mint oil into the glass and add a few slices of lime.
Pour 50ml of Elderflower Gin Liqueur over the mint, ice and lime and add a dash of sparkling water.
Stir the mixture briefly then top with Prosecco.
If you are looking for something stronger you can add a shot of gin.