To make the syrup, combine all the ingredients in a saucepan and bring to a boil.
Reduce the heat and simmer for 10 minutes until the cranberries are soft and the mixture has reduced slightly.
Remove from the heat and strain through a sieve, pushing down on the cranberries with the back of a spoon to get out all the juice. Allow the syrup to cool completely.
Christmas Cranberry Sangria
Fill a large jug with ice and pour over the cranberry syrup.
Add the orange slices and rosemary then top with the Prosecco and soda water and serve.