Soak the dried mushrooms in 100ml hot water for 10 mins until soft, then drain, reserving the liquid, roughly chop and set aside. Season the steaks with lots of pepper and a little salt. Heat the butter and oil in a frying pan until foaming, then cook the steaks for 2 mins on each side for medium rare or longer to your liking. Remove from the pan and keep warm.
In the same pan, fry the shallot, then add the drained mushrooms and cook until lightly browned. Splash in the Sherry, then add the porcini liquid, balsamic vinegar and the cream, and simmer down to make a thick, creamy sauce. Put the steaks and their juices back in the pan, spoon over the sauce and serve with mash.