Let the clarified butter get hot in a large pan. Fry the fillets on both sides, season with salt and pepper and move to the edge.
Put the chopped onions and mushrooms in the middle of the pan and sauté briefly. Stir in the mustard. Place the steaks in the middle and flambé with the cognac. Deglaze immediately with the red wine, add the stock and caramel. Top up with cream and bring to the boil briefly. Arrange on preheated plates.