Peel onions and chop finely.
Place the oil in a large frying pan with a lid and add the onions. Stir and cover.
Sauté onions until translucent, stirring occasionally. The onions should not take on any color.
Add the sugar and cook until all the liquid has evaporated.
Add the balsamic vinegar, stirring gently from time to time until no liquid remains.
Add the cinnamon and let it reduce. The confit is ready when the onions are golden brown and there's no liquid left, only fat.
Place in jars and store in the refrigerator.