Heat half of the butter in a pan and fry the kidneys in portions for approx. 3 minutes while stirring, remove and season with salt.
Also heat the remaining butter, fry the onions and celery in it until translucent, add the mushrooms and sauté. Pour the flour on top, mix in the mustard, pour in the cream and bring to the boil. Add the kidneys with juice, heat and season with salt and vinegar.