Add the mint leaves to a champagne flute or wine glass. Press them gently with a bar spoon to release their oils.
Pour the elderflower syrup into the glass. Fill the glass halfway with ice cubes.
Add the chilled prosecco slowly to prevent overflow, then top with soda water.
Stir gently with a bar spoon to combine all ingredients without losing carbonation.
Squeeze the lime wedge into the drink and drop it in.
Garnish with a mint sprig and apple slice if desired.
Notes
Store elderflower syrup in the refrigerator after opening. It will stay fresh for about 1 month.Pre-chill the prosecco and soda water for best results. Warm sparkling wine loses bubbles faster.Fresh mint leaves work better than dried ones. Pick leaves that look bright green without brown spots.You can make a non-alcoholic version by replacing prosecco with alcohol-free sparkling wine or extra soda water.