Chop the onion finely and fry it in the pot in which you intend to boil the soup. Add the cleaned and sliced mushrooms and fry them for 5-10 minutes, stirring constantly. Without removing from the heat, add the sifted flour, broth and boil for about 30 minutes on low heat.
Remove the sprigs of the parsley and celery, set aside some of the broth and the blend the rest, then return the set aside broth. From the yolks and cream, make a thickening agent and whisk it very well, pour in some hot soup to it and then pour it into the main part of the soup in a thin trickle.