1Pour champagne into a shallow metal pan or bowl, and stir in the sugar. Cover with plastic wrap, and place in freezer. Freeze for 4 hours, whisking every 30 minutes. The frozen mixture will be firm and granular.
2Spoon the sorbet mixture into a blender or food processor, and process until smooth. Return to the metal container, cover, and re-freeze for up to 48 hours.
3Combine strawberries and blueberries in a small bowl. Spoon berries into the bottom of champagne flutes or wine glasses, and top with sorbet. Garnish with mint sprig if desired.