The night before preparing the soup, soak the beans.
Put the beans in a pot with water, bring to boil and cook for about an hour (until they become quite soft). Drain the beans keeping the cooking water.
In another pan, pour oil and heat the onions for some minutes.
Add two spoonful of broth, the bay leaf and the rosemary and cook for about an hour. Keep adding broth being careful not to let evaporate it completely.
Add the beans and one ladle of their cooking water to the onions.
Stir on fire for some minutes then serve with toasted croutons and a drizzle of olive oil.