Thinly slice the onions and chop the mushrooms.
Pat-dry the steak, season with salt.
Heat some oil in a frying pan and cook the steak to your liking, turning every 30 seconds or so. Put them on a plate, cover and let rest.
Add a Tbspful of olive oil to the pan and sauté the onion and mushroom. Add the garlic, stir 30 seconds. Add the mustard, Worcestershire sauce and the meat juices from the plate. Pour in the wine, turn up the heat and cook until the sauce has reduced into a thick, creamy consistency.
Serve the steaks with the hot mushroom sauce poured on top, then season with salt and pepper to your liking.