Make sure your steak is room temperature before you start, and get a heavy pan on very hot. If your steak is 1 inch thick or more get your oven on very hot too.
Salt your meat generously, add a little oil to the pan and cook on one side for a minute. After that flip it over and cook for another 30 seconds. Add some butter to the pan and start basting for 30 seconds.
Start testing the temperature of the meat and once it is within 5 degrees of your desired temp (55C for medium rare) remove it. If it is starting to colour more than you'd like and the temperature is not there, transfer to the oven for a few minutes and continue to test. Rest the meat for 5 minutes before serving.