Prepare the Steak: Remove the steak from the refrigerator and let it sit at room temperature for about 20-30 minutes. Pat dry with paper towels to remove excess moisture.
Season: Rub the steak with olive oil, then generously season both sides with salt, pepper, and your preferred seasonings.
Heat the Skillet: Preheat a cast iron skillet over high heat until very hot (a drop of water should sizzle and evaporate instantly).
Sear the Steak: Place the steak in the hot skillet and cook for 3 minutes, until a golden-brown crust forms. Flip the steak and cook for an additional 3 minutes.
Add Flavor: Reduce the heat to medium. Add the butter, smashed garlic cloves, and rosemary sprigs to the skillet. Allow the butter to melt, then tilt the pan (using an oven mitt) to swirl the butter, garlic, and rosemary around the steak. Spoon the melted butter over the steak continuously while cooking.
Cook to Desired Doneness: Continue cooking until the steak reaches your preferred internal temperature (135°F/57°C for medium-rare or 140°F/60°C for medium, as the temperature will rise about 5°F/3°C during resting). Use an instant-read thermometer for accuracy.
Rest the Steak: Remove the steak from the skillet and transfer it to a clean cutting board. Tent loosely with aluminum foil and let it rest for 5-8 minutes to allow the juices to redistribute.
Slice and Serve: Slice the steak against the grain and serve immediately.