Peel the onions and chop them into small cubes, allow to translate in hot oil. Now dice the Leberkäse and add to the onions, fry a little. Add the glass of mushrooms along with the liquid. Bring to the boil, then add the stock and stir in the mustard. Bring to the boil again and add the cream, then season with salt and pepper. Finally, thicken a little with cornstarch if necessary.