Heat the olive oil or butter in a large saucepan over medium-low heat. Cook the onions with the salt until they start to caramelize, 10 to 15 minutes. Add the plantains or bananas and cook 10 minutes longer. Stir in the garlic, oregano, thyme, and rosemary and cook just until aromatic, about 3 minutes longer.
Add the tomatoes, bread chunks and chicken or vegetable stock. Bring to a boil, reduce to a simmer and cook, stirring frequently, until the bread has softened and the flavors are well blended, about 45 minutes. Ladle the soup into bowls, then squeeze a wedge of lime into each and toss in the wedge. Serve hot.
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