Pat the steaks dry with paper towels and season liberally with salt and ground black pepper. If time permits, let the steaks sit at room temperature for 15 to 30 minutes.
Heat a large cast iron skillet (or heavy stainless steel skillet) over medium-high heat until hot. Add the oil and swirl the skillet to coat the bottom. When the oil is hot, add the steaks to the skillet.
Sear the steaks on the first side until a nice brown crust has formed, about 4 minutes, using kitchen tongs flip the steaks and cook for about 3 additional minutes. Holding each steak with kitchen tongs, quickly turn each steak on its sides, to sear the edges (about 1-2 minutes).
Reduce the heat to medium low and add the butter, rosemary, and garlic. Tilt the skillet so the butter pools to one side of the skillet. Spoon the butter over the steaks and continue to cook until the steaks are about 5-10 degrees away from the desired doneness. (The steaks' temperature will continue to rise 5-10 degrees while they rest).
Remove from the heat and transfer to a cutting board or plate and loosely tent with aluminum foil. Let the steaks rest for 10 minutes before serving.
Serve topped with herb butter (if desired), or spoon some of the butter from the skillet over the steaks.