Cut excess fat from the steaks and make small incisions around the outside wherever there is a line of gristle (between the fat and the meat). This prevents curling. Over medium-high heat, bring the oil to the smoking point. Saute the steaks on one side for 2 minutes. Turn the steaks with tongs or a fork inserted close to the rim of the fat, and saute 2 to 3 minutes. Immediately remove the steaks to a warm platter and season with salt and pepper. Cover loosely with foil while making the sauce. Discard any fat in the pan. Add the cognac and scrape up the coagulated juices. Add the cream and reduce by 1/2. Stir in the mustard and pour the sauce over the steak.