In a heavy-duty sealable plastic bag or between 2 sheets of waxed paper crush peppercorns and fennel seeds coarsely with bottom of a heavy skillet. Pat steaks dry and coat both sides with salt and the 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. Transfer to plates and cover to keep warm.
Pour off excess fat from skillet and add shallots and 1/2 cup broth and deglaze pan. Add cream and Cognac. Boil mixture, scraping up browned bits, until sauce thickens and coats back of spoon, about 1 minute. Finish sauce by whisking in mustard. Taste and season sauce with salt, if necessary and spoon over steaks.