Sprinkle your steak generously with salt and if it came from the fridge, let stand at room temperature for 10-15 minutes to take the chill off.
Heat a drizzle of oil in a heavy skillet (cast iron is ideal) set over medium-high heat, and once the pan is hot, cook the steak for 5 minutes, until it develops a deep bottom crust.
Flip and cook for another 5-6 minutes for medium-rare, or until done to your liking. (Thicker steaks may require a bit more time. If the oven is on, you could slide the whole pan into the oven for 10 minutes or so after flipping.)
Transfer to a cutting board and let rest for 10 minutes.
Add the butter to the pan and cook the mushrooms for 4-5 minutes, scraping up any browned bits in the bottom of the pan, until the mushrooms are soft and starting to turn golden.
Serve the steak topped with mushrooms. (If you like, add a splash of wine, beer, stock, cream or even water to the pan to get the last of the flavourful bits, and dribble the pan sauce over the steak as well.)