In a large heavy pot, heat oil over medium heat. Cut onions in half lengthwise, then 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 color, 30 minutes.

Increase heat to medium and cook, stirring occasionally, uncovered, until onions are amber in color, 45 minutes.

Sprinkle onions with sugar and a little bit of salt; cook, stirring, until sugar melts and onions are caramel colored, 5 minutes. Pour in wine and broth, and place cinnamon stick in pot. Bring to a boil, then reduce heat, cover and simmer 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. Whisk the soup to incorporate the bread. Adjust seasoning. Serve hot.