Prepare the fruit. Rinse the cranberries and pick out any that are soft or damaged. If you are using frozen berries, leave them frozen. Dice the apple into small, even pieces so it cooks through at the same rate.

Start the sangria base. Combine the sangria, sugar, orange juice, apple, cinnamon stick and salt in a medium saucepan. Bring it to a gentle boil over medium heat, stirring until the sugar dissolves, then lower it to a simmer and cook 5 minutes.

Cook the cranberries. Stir in the cranberries and simmer uncovered 12 to 15 minutes, stirring now and then, until most of the berries have burst and the apple is tender. Mash some of the berries against the side of the pan and leave the rest whole. It should still look a little loose, because it thickens as it cools.

Finish and balance. Take the pan off the heat, discard the cinnamon stick and stir in the orange zest. Taste a cooled spoonful. The sangria is already sweet, so add more sugar only if it needs it, a tablespoon at a time while the sauce is still warm.

Chill. Transfer to a covered container and refrigerate at least 2 hours. Serve it cold, or let it sit out briefly for a softer texture.