Make an x on the bottom of each peach. Plunge into boiling water for 2 minutes, then transfer to an ice water bath for 1 minute. Peel skin from peaches - it will slip right off! Pit peaches and put peach pieces in a blender. Add lemon juice and sugar and blend until smooth. Strain through a sieve. Chill until ready to use.
To serve, combine 1 part peach puree with 2 parts sparkling wine. You won't be able to pour all the champagne at once due to the bubbles. Stir to combine after the first pour, then top with more bubbly.