Sprinkle steak with salt and pepper. Let stand at room temperature for 30 minutes.
Heat a large cast-iron skillet over high heat. Add oil, and swirl pan to coat. Add steak; cook until browned, about 3 minutes per side.
Reduce heat to medium-low, and add butter, rosemary, and garlic. Carefully tilt pan toward you so butter pools; cook steak for 1½ minutes, basting with butter mixture constantly. Transfer steak to a cutting board, and cover loosely with foil. Let stand for 10 minutes. Reserve butter in skillet, discarding rosemary and garlic.
Thinly slice steak diagonally across the grain; top with butter. Garnish with rosemary, if desired.
Recipe by Paula Deen Magazine at https://pauladeenmagazine.com/cast-iron-seared-steak-recipe/