How to Make It
Preheat the oven to 200°. Pepper both sides of the steaks.
In a large heavy nonreactive skillet, heat the oil over high heat. When it begins to smoke, add 2 of the steaks and sear them for 2 minutes on each side. Transfer to a heatproof plate. Keep warm in the oven and repeat with the remaining 2 steaks.
Discard the oil. Add the butter to the pan. Add the onion and cook over high heat, stirring, until cook over high heat, stirring, until browned, 2 to 3 minutes. Add the wine and 1 cup of the stock and boil over moderately high heat until the liquid is reduced to 1/2 cup, about 4 minutes.
Dissolve the arrowroot in the remaining 1 tablespoon of stock and stir into the sauce. Remove the pan from the heat, add the steak juices and season with salt and pepper. Spread the onion sauce on warmed plates, top with the steaks and serve at once, garnished with the parsley.