Remove the steak from the refrigerator and let it sit at room temperature for at least 30 minutes before cooking to ensure an even sear. Pat dry with paper towels and season generously with kosher salt and black pepper.
Heat a cast-iron skillet over medium-high heat until hot. Add a small amount of olive oil to coat the pan. If the steak has a fat cap, sear the fat side first to render some of the fat.
Sear the steak for about 4 minutes on the first side, Flip and cook for an additional 2–3 minutes for medium doneness.
Reduce the heat to medium-low and add the butter, rosemary, and garlic cloves. Tilt the pan slightly and use a spoon to baste the steak with the melted butter for about 1 minute, or until the desired doneness is reached.
For accuracy, use a meat thermometer: 130°F for medium-rare, 140°F for medium, and 150°F for medium-well.
Transfer the steak to a cutting board and let rest for 5–10 minutes before slicing into bite-sized pieces.