For the bean soup, in a saucepan fry the finely chopped onion with a little oil and the flour (until the quantity has turned brown).
Meanwhile, drain the beans and rinse them with water. Add about 3/4 liter of water to the onion-flour mixture and bring to a boil.
If the thickness of the bean soup is not to your liking, add more water. Then empty the soup through a sieve into another pot and add the beans.
Season with salt and pepper, bring to the boil again and serve the bean soup.