Pat steak dry with paper towels, then season liberally on both sides with salt and pepper. Heat a large cast-iron skillet over high heat. When pan begins to smoke, coat with oil, then add steak (be careful; oil will spatter) and sear, until almost black, for about 4 minutes on each side (for rare). Don't move meat until ready to turn or crust will break. Transfer steak to a platter and top with garlic butter. Cover loosely with foil to keep warm. Allow steak to rest for 10 minutes, then cut across the grain into 1/4" slices. Spoon juices over steak and top with parsley.
GARLIC BUTTER: Heat butter and garlic in a small saucepan over medium-low heat. Cook until garlic is fragrant, about 5 minutes. Remove and discard garlic. Keep butter warm. Makes enough to top 4 servings of steak.