Granita is very easy. For this champagne granita, you need to make a raspberry syrup base. Then, you just cool the syrup, mix with champagne and freeze it for an hour before stirring and scraping.
In a small saucepan, mix the water, sugar, and raspberries. Heat on high, mashing the raspberries so that they infuse the syrup. Bring the mixture to a boil, then take it off the heat and strain it into a large heatproof bowl. Chill for 1 hour or until completely cooled.
Combine the raspberry syrup (all of it) with the bubbly and lemon juice. Transfer to a 9×13″ Pyrex and freeze for 1 hour.
Stir the slushy granita base with a fork, scraping down the sides and mashing any big clumps of ice. Put the pan back in the freezer, stirring it every 30 minutes.
Continue stirring and freezing for about 2 hours or until you have what looks like shaved ice. Keep the granita frozen until you are ready to serve