First cut the peach fruit off the pit. Purée in blender and strain. Toss the leftovers in the strainer and chill the strained fruit.
Measure 3/4 of an ounce of the peach purée and combine with the St. Germain, vodka, simple syrup, lemon juice in a chilled cocktail mixer full of ice. Shake. Pour into cocktail glasses. Top the drinks with prosecco and garnish with rosemary.