Make one fruit purée at a time: In a blender, combine the fruit and sugar, and (if using) the brandy or lemon juice. Blend to a smooth purée. Taste and add more sugar if needed; the purée should be sweet but not overly sweet. Pour the purée into a small pitcher or carafe, straining it through a coarse sieve if a smooth consistency is desired.
Fill 6 Champagne flutes about one-third full with the fruit purée. Slowly fill the flutes with the sparkling wine. Stir briefly and serve, garnished with fresh fruit.