Heat the olive oil in a large pan over low medium-to-high heat and fry the onions, stirring occasionally, for 5 minutes until golden. Add the stock and bring to the boil. Reduce the heat and simmer for 10 minutes.
Meanwhile, toast the bread and cut into croutons.
Check the soup for seasoning and serve in bowls, topped with the croutons. Serve with sliced French bread that has been toasted and topped with grated cheese for a traditional accompaniment.
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