In a large non-stick or cast iron pan heat the olive oil over high heat until shimmering. Pat the beef dry and season with sea salt. Place the beef in the hot pan and sear undisturbed on each side for about 6 minutes or until the internal temperature reaches your desired temperature according to the temperature guide above.
Set the beef aside to rest and cover with foil to keep hot.
While the beef is resting, make the sauce in the same pan as you seared the steak. Place the pan over medium heat and add the shallots, mushrooms and garlic and sauté until the shallots are soft and the mushrooms are golden. Remove them from the pan into a small bowl.
Add the white wine, rosemary and time and reduce the wine by half. Pour in the cream and mustard and whisk well. Bring the sauce to a simmer and add the mushrooms back into the sauce. Place the beef and any of the juices on the plate into the pan and serve.