Preheat oven (electric cooker: 225°C/circulating air: 200°C/gas: level 4). Line a baking tray with baking paper and spread the breads on it. Peel and finely dice the onions. Clean the mushrooms, wash if necessary and cut into slices.
Wash the meat and dab dry. Heat the oil in a large pan. Fry the meat for about 3 minutes on each side. Dice the gorgonzola.
Fry the onions and mushrooms in hot frying fat for 3-4 minutes until brown. Season with salt and pepper. Dust with flour and sauté briefly. Deglaze with wine
Stir in sour cream and gorgonzola. Season to taste with salt and pepper.
First spread Kasseler, then mushroom cream on the bread. Bake in the oven for about 5 minutes. In the meantime, wash parsley, shake dry, chop and sprinkle on top.