Toss the steak with 1/4 tsp of the salt and the pepper.
Heat the butter in a large skillet over high heat until it's blazing hot, then add the steak in small batches
Sear the steak, turning as soon as a dark brown crust appears, which, as long as the skillet is hot enough should take 30 seconds per side. Turn steak so all sides are seared. Remove from the skillet and set aside. Repeat with all the steak
Add the mushrooms to the skillet, reduce the heat to medium and saute for 5 minutes until the mushrooms release their juices
Add the beef stock, vinegar, parsley and remaining salt to the skillet. Cook for 5 minutes or until the stock has reduced by half and slightly thickened. Pour the mushrooms over the steak and serve