Season the steaks generously with salt and pepper. Heat a large, heavy skillet (such as cast iron) over medium-high heat and add the oil. When the oil is hot, add the steaks, and sear for about 4 minutes on each side, until nicely browned on the outside and cooked to your liking, about 125°F internal temperature for rare, 135°F for medium-rare. Transfer them to a cutting board, and tent them with foil. Pour off any remaining fat from the skillet, but do not clean the skillet.

Melt the butter in the same skillet over medium-low heat. Sauté the shallots for 4 minutes, until golden brown and tender. Add the mustard, Worcestershire sauce, red wine and cognac (be careful, the liquor can ignite) and stir, scraping up any little browned bits from the bottom of the pan. Allow the sauce to reduce a bit, just one minute, then taste and season as needed. Stir in the parsley.

Slice the steaks and transfer them to a serving plate, or serve each steak on an individual plate with the desired side dishes. Drizzle the sauce over the steak and serve.