Mix vinegar, oil, mustard, salt and pepper, rub the steaks well with this marinade and leave for an hour.
Heat the clarified butter in a frying pan and roast the steaks for 5 min on each side over medium heat. In the meantime, dice the onion. Remove the steaks and keep warm. Deglaze the meat stock with a little clear soup, sauté the onions until soft, add the mushrooms and the rest of the marinade. If necessary, add a little more clear soup. Simmer slightly and pour over the finished steaks form.
Serve with Brussels sprouts and mashed potatoes.