Muddle the mint in a cocktail shaker.
Add about 0.5 ounces of fresh lime juice and 1.5 ounces of elderflower liqueur to the cocktail shaker as well. Mix well with the crushed mint leaves. Save the rest of the lime for garnish.
Garnish the glass: add thin slices of lime and a couple sprigs of mint. You can add a lime wedge to the edge of the glass as well, if you'd like.
Cover the cocktail shaker with the strainer top, and pour the liquid into your wine glass, making sure to prevent the muddled mint leaf debris from getting into the glass.
Pour 6 ounces of prosecco and 3 ounces of club soda into the wine glass. Stir well.
Add ice, stir, and serve immediately.