Step 1

Let steak stand at room temperature for 30 minutes. Pat dry with paper towels. Brush with oil and sprinkle with 1/2 teaspoon salt and 1/4 teaspoon pepper.

Step 2

Heat a large cast-iron skillet over medium-high heat. Add steak and cook for 1 minute. Add garlic, shallots, butter and rosemary; cook, tilting the pan slightly to pool the butter and drippings on one side, for 1 minute. Use a spoon to baste the steak with the butter mixture. Flip the steak and cook, basting occasionally, until an instant-read thermometer registers 125 degrees F for medium-rare, 2 minutes more. Transfer the steak to a clean cutting board and top with the shallots, garlic and rosemary. Cover loosely with foil; let stand for 10 minutes.

Step 3

Discard the rosemary sprig. Thinly slice the steak against the grain and transfer to a serving platter. Spoon drippings from the pan and cutting board over the steak. Season with the remaining 1/4 teaspoon each salt and pepper and garnish with more rosemary, if desired.