Add the water and raspberries to the blender, and blend until smooth. Strain the previous mixture with a strainer, to separate the seeds from the mashed raspberries. Discard everything that remains in the strainer.
Put the mashed raspberries in the blender again, adding sugar and mint leaves. Blend until the sugar dissolves.
Put the mixture in the freezer for at least 2 hours.
To serve, first put 2 tablespoons of sorbet in each cup, then the champagne.
