Using a mortar and pestle or the bottom of a heavy skillet, crush the whole peppercorns until coarse. Pat steaks completely dry with paper towels. Season generously on all sides with kosher salt, then press the crushed peppercorns firmly into the top and bottom of each steak.

Place a skillet over high heat and add oil. When shimmering, carefully place steaks in the pan. Sear for 3-4 minutes per side for a deep brown crust. Reduce heat to medium, add butter, and baste the steaks for 1 minute. Cook to an internal temperature of 130°F (54°C) for medium-rare. Remove steaks to a cutting board to rest.

Reduce pan heat to medium-low. Add the minced shallot and sauté for 1-2 minutes. Remove pan from heat, add cognac, then return to heat and scrape up any browned bits, reducing the liquid by half. Stir in beef broth and Dijon mustard, simmer for 2-3 minutes. Stir in heavy cream and simmer for 4-5 minutes until thickened. Season with salt if needed.

Pour any accumulated juices from the steak's resting board into the sauce. Slice the steak into thick strips against the grain. Arrange slices on a plate, spoon a generous amount of sauce over the top, and garnish with finely chopped fresh parsley.