Rinse the cranberries under cold water and pick out any soft or damaged ones.
In a medium saucepan, combine the cranberries, sugar, orange juice, orange zest, and cinnamon (if using).
Bring the mixture to a boil over medium-high heat, stirring occasionally to dissolve the sugar.
Reduce heat to medium-low and simmer for 10-12 minutes, stirring occasionally. You'll hear the cranberries start to pop—that's exactly what you want!
Once the sauce has thickened and most of the cranberries have burst, remove from heat. It will thicken even more as it cools, so don't worry if it looks a bit loose.
Transfer to a serving bowl or airtight container and let cool to room temperature before refrigerating. The sauce will keep for up to a week in the fridge.
