Heat the oil in a pan. Season the steaks, then fry for 2-3 mins each side for medium rare, or cook more to your liking. Remove from the pan, then set aside to rest.
Add the mushrooms and thyme leaves to the pan, then cook for a couple of mins until softened and golden. Pour in the wine and bubble until syrupy, then turn off the heat and stir in the butter. Season.
Serve the steaks with the mushroom sauce poured over, along with creamy mash and some crunchy spring greens.