Place the foie gras onto some Clingfilm and roll up into a cylinder shape and chill in the fridge for about 30 minutes.
Heat the butter and olive oil in a large, heavy-based frying pan. When the butter foams, add the beef and fry for 3-4 minutes on one side, then flip over and fry for another 3 minutes on the other side. When ready the steaks should be crisp on the outside, but rare inside. Season them with salt and freshly ground pepper and set aside in a warm place, to let the meat rest.
Cut the chilled foie gras into 6 slices and sear in the steak pan until caramelised on each side. Remove from the pan and keep warm. Meanwhile slice the mushrooms and sauté in the pan until soft, remove the mushrooms and de-glaze the pan with the red wine and cook for one minute, then add the stock and reduce the liquid by at least half, season well.
Toast the bread until golden and place on the serving plates. Put the steak on top of the toasted bread and put the seared foie gras on top. Sprinkle some mushrooms around the dish and drizzle with the red wine jus. Lastly garnish with some black truffle shavings.