Preheat oven to 200°C/400°F/Gas 6. Generously season the steaks on both sides with salt and black pepper, to taste. Set aside.

Add olive oil to a large cast-iron skillet set over medium-high heat. Heat skillet for 5 minutes until very hot, before adding the meat. Sear the steaks until nicely browned, approximately 2-3 minutes per side.

Remove skillet from heat and top each steak with 1 tablespoon unsalted butter.
Transfer the skillet to the center of the preheated oven and roast for 8-10 minutes or until an instant-read meat thermometer reads 130°F/55°C for medium-rare.*

Once the steaks are in the oven, add the remaining butter to another skillet set over medium heat. Add mushrooms, rosemary, and garlic powder. Season with salt and black pepper, to taste, and stir to combine. Cook for 5-6 minutes, stirring frequently until the mushrooms are nicely browned and start to release their juices. Remove from heat and set aside.

Remove skillet from oven and transfer steaks to a large plate. Cover loosely with aluminum foil and rest for 5 minutes. (Internal temperature will continue to rise several degrees while they rest). Remove foil and serve immediately topped with some of the sautéed mushrooms. Enjoy!