In a heavy-duty seal-able plastic bag or between 2 sheets of waxed paper, crush all peppercorns and fennel seeds coarsely, using the bottom of a heavy skillet. Pat steaks dry and coat both sides with peppercorn mixture.
In a 10-inch heavy skillet, heat butter and oil over moderate heat until hot but not smoking and cook steaks for 4 to 5 minutes on each side for medium rare. Season steaks with salt and transfer to plates.
Pour off excess fat from skillet and add cream and Cognac. Boil mixture, scraping up browned bits, until sauce thickens and coats back of spoon, about 1 minute. Season sauce with salt and spoon over steaks.