1bottle (750 ml)champagne, sparkling white wine or prosecco,chilled
Before you get started, clear a space in your freezer for your baking dish to sit flat and level.
In a saucepan, combine the grapefruit juice and sugar. Heat the juice over medium heat, stirring just until the sugar is completely dissolved. Remove from the heat, and chill the juice for several hours until well chilled.
Open the champagne and pour into your baking dish. Add the chilled grapefruit juice and sugar mixture to the champagne and stir to combine. Cover the dish with plastic wrap and freeze for 1 hour.
Use a fork to scrape the ice crystals that are forming around the edge of the champagne mixture into the center of the dish. Return the dish to the freezer and continue to scrape and stir it with a fork every 1 1/2 hours for the next 8-10 hours. As it freezes and you continue to break up the ice crystals with a fork, the granita will go from liquidy to slushy to light, fluffy and sparkly.
Once the granita is fluffy, you can spoon it into chilled glasses and serve it right away, or leave it in the freezer to serve the next day.
Store the granita in an airtight container in the freezer for up to 2 weeks.