Let steak stand at room temperature for 30 mins. Pat dry with a paper towels. Brush both sides with oil and sprinkle salt and pepper on each side.
Heat a large cast iron skillet over medium high heat. Add steak and cook for one minute. Add garlic, onions, butter and rosemary; Cook, tilting the pan to pool butter and drippings to one side for 1 minute
Use a spoon to baste the steak with the butter mixture. Flip the steak and cook basting occasionally, until a instant read thermoter registers 125° F. For medium rare, cook for 2 minutes more.
Transfer the steak to a clean cutting board and top with the onions, garlic and rosemary. Cover with tinfoil and let it stand for 10 minutes.
Discard the rosemary, thinkly slice the steak against the grain and stransfer to a serving platter. Spoon drippings from the pan and cutting baord over the steak.
