Combine diced peach, raspberries and water in a blender pitcher; blend until smooth. Pour puree into a measuring pitcher through a fine sieve to remove seeds. Use a rubber spatula to help the puree move through the sieve. (If possible, cover and chill for an hour or more before assembling cocktails.)
Pour ~1 1/2 ounces of fruit puree into a flute glass; top with a little Prosecco and gently stir. Once foam has subsided top off with more Prosecco. Serve.
Note that foam can be an issue when making these cocktails. Starting with cold ingredients is helpful. If possible, make your fruit puree and chill for hour or more before making cocktails. Opening your Prosecco (but keeping it in the refrigerator) 10 to 15 minutes beforehand also reduces foam.