This traditional Italian dish, also called Cipolline Agrodolce is usually served with roasted meats like porchetta.
Melt 2 tablespoons butter in a saucepan over medium heat. Add 1 each sliced Vidalia onions and red onion, cook stirring until soft about 10 minutes. Add ¼ cup ketchup, 1 tablespoon red wine vinegar, 2 teaspoons sugar, ½ teaspoon hot sauce, salt and pepper to taste and heat for 3 more minutes. Stir in 3 chopped scallions.