In a large, heavy pot, heat oil over medium heat. Peel onions, then cut in half lengthwise, and slice thinly. Place sliced onions in pot and toss to coat with oil. Cover, reduce heat to medium-low, and cook, stirring occasionally, until onions are softened and beginning to colour, about 30 minutes.
Uncovered, increase heat to medium and cook, stirring occasionally, until onions are amber in colour, about 45 minutes.
Sprinkle onions with sugar and a little salt. Cook, stirring, until sugar melts and onions are caramel-coloured, 5 minutes. Pour in wine and broth, and add cinnamon stick to pot. Bring to a boil, then reduce heat, cover and simmer for 1 hour.
Stir the bread into the soup and cook, uncovered, stirring occasionally until the bread disintegrates, 30 to 45 minutes.
Remove the cinnamon stick and whisk the soup to incorporate the bread. Adjust seasoning. Serve hot.