Pat the steaks completely dry with paper towels.
Just before cooking, season both sides generously with salt and pepper.
Heat a cast-iron pan until hot. Add the oil over medium-high heat and swirl to coat.
When the oil is very hot, add the steaks to the skillet.
Sear the first side for 4 minutes, until a brown crust forms.
Flip and cook the second side for another 3–4 minutes.
Using tongs, turn each steak onto its sides to render the fat and sear the edges, about 1 minute per edge.
Reduce the heat to medium and immediately add the butter, garlic, and rosemary.
Tilt the pan and continuously spoon the melted garlic butter over the steaks for about 1 minute.
Continue basting until the steaks are just below the desired doneness.
Transfer the steaks to a cutting board and loosely cover.
Rest for 10 minutes, then slice into strips against the grain.
Spoon any extra butter sauce over the sliced steak before serving.