Combine the red onion, cranberries, chopped oranges, orange juice, minced ginger, orange peel, cinnamon, and brown sugar in a medium saucepan.
Bring the mixture to a boil over medium-high heat, stirring occasionally.
Reduce the heat to low, cover the saucepan, and let it simmer for 15 to 20 minutes.
Continue to simmer until the cranberries have burst and the relish has thickened to your desired consistency.
Let cool before serving.
Taste and adjust the sweetness at the end of cooking. If it's too tart, add a bit more sugar; if too sweet, a squeeze of fresh lemon or orange juice can brighten it up.