Season steak liberally with salt and coarsely ground black pepper, then flip and season the other side. Set aside until ready to cook, at least 30 minutes, better an hour if you have it.
Preheat the oven to 400℉/200℃.
Add 1-2 tablespoons of oil to a cast-iron skillet and tilt around until the surface is lightly covered with oil. Place over high heat until super hot, and it starts to smoke.
Place steak into the hot skillet and sear for about 1-2 minutes or until it browns and forms a crust. Flip and do the same on the other side.
Reduce the heat if the meat is browning too quickly. If you prefer a lighter outside crust, then slightly reduce the time.
Turn off heat, add butter and rosemary, put it in the preheated oven, and continue cooking for about 7-10 minutes.
Use an oven pan mitt to tilt the pan so butter goes to one side of the pan, spoon butter over fillet steaks, and cook until it reaches desired doneness.
This will vary greatly depending on the thickness of the steak and preference.
Use an instant-read thermometer to gauge temperature.
Pour any remaining juices and butter from the pan over the steak and sprinkle with parsley before serving.