Place peach slices in a blender and blend until smooth.
Strain the peach puree through a fine mesh strainer to remove any pulp.
Add lemon juice to the strained puree and mix well. Add sugar if desired.
Pour 2 tablespoons of peach puree into each champagne flute.
Fill the glasses with chilled champagne, pouring slowly to prevent overflow.
Stir gently with a long spoon to combine the puree and champagne.
Garnish with fresh peach slices.
Notes
The peach puree can be made up to 24 hours ahead and stored in the refrigerator.Use ripe, yellow peaches for the best flavor. White peaches tend to be less sweet.To prevent the cocktail from being too sweet, choose a dry champagne or prosecco.One bottle of champagne makes about 6-8 cocktails, depending on glass size.Keep all ingredients chilled for the best results. Warm champagne will create excess foam.