Add peach slices to a blender with lemon juice and sugar. Blend until smooth.
Strain the peach puree through a fine-mesh strainer into a bowl. This removes any pulp.
Fill each champagne flute 1/4 full with the peach puree.
Tilt the glass slightly and slowly pour cold Prosecco until the glass is full.
Stir gently with a bar spoon to combine the puree and Prosecco.
Add garnish if desired and serve right away.
Notes
Store extra peach puree in an airtight container in the fridge for up to 2 days.Frozen peaches work well when fresh ones aren't in season. Thaw them completely before blending.The sugar amount can be adjusted based on the sweetness of the peaches.For best results, chill both the peach puree and Prosecco before mixing.