Cook steaks and keep them in one layer in skillet. Remove steaks from skillet and skim all but 1 tablespoon of the fat from the pan juices in the skillet. To skillet add, shallots and minced garlic and cook them for 1 to 2 minutes until softened. Deglaze with Cognac, scraping up bits which cling to skillet. Add stock or canned beef broth and peppercorns. In a small bowl combine cup heavy cream and cornstarch. Whisk cornstarch mixture into sauce and simmer until lightly thickened. Whisk in mustard and pour juices from plate into pan.