How to Make It
Set a 12-inch cast-iron skillet over high heat. Season the steaks generously on both sides with pepper. When the skillet is searing hot, season the steaks with salt, add them to the skillet and cook for 2 minutes. Shake the skillet; the steaks should be seared enough to loosen. Using tongs, turn the steads and cook until browned on the second side but still quite rare, about 2 minutes. Shake the skillet again to loosen the steaks, then transfer them to ovenproof plates and keep warm in a low oven.
Immediately add the butter and onion to the skillet and cook over moderate heat, shaking often, until the onion is softened, about 5 minutes. Add the wine and bring to a boil over high heat, scraping up any browned bits. Using a slotted spoon, transfer the onion to the plates. Boil the sauce until it is reduced and tasty; season with salt and pepper. Pour the sauce over the steaks and serve immediately.