In a medium saucepan set over medium heat, combine the cranberries, sugar, orange juice and zest, Grand Marnier and 1 cup water.
Bring the mixture to simmer and cook it until the cranberries burst and soften, about 15 minutes.
Remove the mixture from the heat and allow it to cool for 10 minutes.
Place a sieve over a bowl then pour the mixture through the sieve. Discard the solids then cover and refrigerate the syrup for a minimum of 2 hours and up to overnight.
When ready to serve, add 2 tablespoons cranberry syrup to a Champagne flute and top it with chilled champagne. Serve immediately.
Notes
As soon as you bring your champagne home, put it in the fridge. Champagne cocktails are best cold, but ice would dilute the flavor too much. Hence, make sure the champagne bottle is properly chilled!
You can make the cranberry syrup up to 5 days ahead and store it in the fridge in an airtight container.
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