Heat the oil and stew the finely chopped onions in it. Add the parsley and gently heat it. Add the mushrooms, cut into thin slices, sprinkle them with salt and pepper and stew for 5 minutes.
Pour in the broth, add the bay leaves and nutmeg, and boil for 3 minutes with a lid on. With a mixer, mix the flour and yoghurt and add them to 1/3 of the soup in a separate bowl.
Blend the mixture well and then mix it with the rest of the soup. Boil for 3 minutes, stirring constantly. Season with the lemon juice and white wine.