Preheat oven to 375 degrees F. Toss onions with sugar and oil in a shallow 2-quart baking dish. Bake, uncovered, until the onions are very tender and golden, 1 to 1 1/4 hours, stirring every 15 minutes.
Transfer the onion mixture to a medium saucepan and add cranberries, orange juice and zest, vinegar and salt. Cook, stirring, over medium heat until the cranberries are tender and the mixture is thickened, 5 to 10 minutes. Let cool.
Make Ahead Tip: Cover and refrigerate for up to 1 week.
1/2 other carbohydrate