in a wide coated frying pan roast the flour on a small (!) Fire leisurely chestnut brown, but not too dark, otherwise the soup will be too bitter. While doing this, stir well with a wooden ladle so that the flour doesn’t burn. Note: this really takes a lot of time, but is absolutely necessary for a good result. Remove the frying pan from the fire and add the soup, stirring with a whisk. Add butter in flakes.
Bring to a boil and do uncovered on a low fire for a good 45 minutes. This is important so that the flour taste disappears.
The soup should now have a thick consistency, add a little more soup as needed. Season with salt and pepper just before bringing to the table. In the meantime, remove the peel from the onions and cut them into fine rings. Fry in oil until light brown. Toast the bread and cut into small cubes. Sprinkle the onion, bread and cheese over the soup just before serving.