In a heavy frypan over medium-high heat, melt the butter and add the olive oil. Add the steaks and sear them without moving them for 2 minutes. Check for dark brown color (they may need more time if they were quite crowded in the frypan). Turn and repeat. Remove steaks from frypan and cover with foil to keep them warm (or put them in a 95 degrees C oven).
Add to the frypan the garlic and saute for about 30 seconds. Add the mushrooms and continue to saute for another minute or two, until the mushrooms have softened and given up some of their liquid.
Add the wine and scrape up any bits stuck to the pan. Add the parsley and stir well. Cook until the majority of the liquid has evaporated (but do not cook dry).
Return the steaks to the frypan and add any juices that may have accumulated on the platter. Cook for 1 full minute. Serve at once.