1. Combine the sugar and orange juice in a large saucepan and bring to the boil.
2. Stir in the cranberries and clementine zest (if using), then simmer until the cranberries are tender but still holding their shape – this will take about 5 minutes if using frozen cranberries and a little longer if using fresh.
3. Refrigerate until needed, the sauce will thicken as it cools.