Peel the onions and cut into thin slices, heat the margarine and let the onions turn golden yellow. Add the flour and sauté briefly. Then deglaze with a little water and fill up with the remaining liquid. Salt the soup and cook over a mild heat. Then season to taste with salt, pepper and nutmeg. Toast the white bread and cut into cubes. Then put the soup in fireproof soup bowls. First put the bread and then the grated cheese on the soup.
Let the soup turn golden yellow in the oven at approx. 200 ° C (this takes approx. 5-10 minutes and depends on the oven) and serve immediately.