STEAK: Trim the steak of all the surrounding fat and cartilage. Cut the meat into 2 pieces and crush the peppercorns using the bottom of a heavy skillet.
Sprinkle salt lightly on the top and bottom of the steaks. Press the cracked peppercorns on each side of the steak, coating lightly or heavily to your taste.
Heat the oil and the butter in one of the pans over high heat. When the pan is hot, carefully lay the peppered steaks in. Fry for about 2 minutes on one side, until well seared. Turn the meat and cook the other side for about a minute. Press your finger into each steak to test. If the steak feels slightly springy, that means it's rare. Cook another minute or two for medium rare. Remove the steaks to a warm platter and cover with foil.
PAN SAUCE: Add the shallots to the pan and sauté briefly, stirring with a wooden spoon to scrape up the drippings. Carefully pour in the cognac, leaning away from the burner. Tilt the edge of the pan slightly over the burner flame to ignite the alcohol. The cognac will flame for a few seconds as the alcohol burns off. Cook for another 30 seconds then add the stock. Bring the liquid back to the boil, and cook about 1 minute to thicken the sauce, stirring occasionally. Taste and adjust seasoning. Add the soft butter, swirling the pan until it melts and incorporates with the juices. Serve over the steaks.
MUSHROOMS: While the steaks cook, melt the butter in the skillet over medium heat. Add the shallots and cook until soft and translucent. Add the mushrooms and saute until the mushrooms soften and glisten. Add the white wine and the balsamic vinegar, stirring and cooking until the sauce reduces by ⅓. Season to taste with salt and pepper to taste. Serve immediately.
VARIATIONS: If you cook on an electric stove or cooktop, you can skip flaming the pan sauce.