Onions peel and chop finely. Clean, clean and slice the mushrooms. Wash the meat and dab dry. Heat oil in two portions in a large pan, fry 4 slices of meat on each side for about 3 minutes.
Remove from the pan and fry the rest of the meat in the same way. Remove from the pan. Sauté the onions in the frying fat and remove from the pan. Fry the mushrooms in three portions in the pan while turning.
Add onions again, season with salt and pepper. Dust with flour, sweat and gradually add cream and stock. Bring to the boil and season to taste with salt, pepper and sugar. In the meantime, roast the slices of bread on a baking tray in a preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for 2-3 minutes.
Cut the cheese into slices. Put a slice of meat on each slice of bread and spread cheese evenly on top. Then spread mushroom cream on top. Bake at the same temperature for another 12-15 minutes.
Wash parsley, shake dry and chop finely. Sprinkle gourmet slices of Kasseler with parsley and garnish.