In a blender or food processor, puree the peach until smooth. If there's chunks of skin (lovely mental image), strain out with a fine mesh sieve and discard. One peach makes enough puree for about 2 drinks.
In a tall glass, stir together 2 oz of the peach puree with the lemon juice and St. Germain. Fill the glass with ice and carefully pour in the prosecco. Top with sparkling water.
Garnish with mint leaves and a peach slice, as desired.
Keyword cocktail, drinks, elderflower, peach, prosecco, spritz, st germain, summer