Put the sugar, water, orange juice and salt in a medium saucepan and bring to a simmer over medium heat, stirring until the sugar dissolves.

Add the cranberries and zest. Bring back to a gentle boil, then reduce to a lively simmer.

Cook 8–10 minutes, stirring occasionally, until most berries have popped and the sauce looks glossy and slightly loose — it will thicken as it cools. A few whole berries remaining is right.

Take off the heat. Taste carefully (it is hot) and adjust: a spoonful more sugar if very tart, a squeeze more orange if it tastes flat.

Pour into a serving bowl or jar, cool to room temperature, then cover and refrigerate at least 2 hours before serving so it sets softly.