1. If using fresh strawberries, freeze until completely solid, about 1-2 hours.
2. In a food processor or blender, add frozen strawberries and sugar. Blend until smooth. Add warm water as needed and press down with a spoon or rubber spatula to help the blades along.
3. Transfer to a freezer-safe container and freeze until solid, about 1-2 hours.
4. Spoon sorbet into a glass and pour champagne over top. Serve immediately. Garnish glass with a fresh strawberry, if desired.