Bring 4 cups of water to a boil. Add the peppercorns and bay leaf.
Briefly and separately blanch the carrot, turnip, and celery root. Refresh the vegetables in ice water and set aside. Reserve the vegetable stock resulting from the blanching and reduce to 1/3 cup. Remove the bay leaf and peppercorns.
Heat 2/3 cup clarified butter in a skillet and sauté the sirloin over medium high heat until brown outside and medium rare inside. Keep warm on a covered plate. Degrease the pan. Heat 1/3 cup clarified butter and sauté the onion over medium heat until golden. Add the cognac and reduce 3 to 5 minutes. Add the vegetable stock and again reduce 4 to 5 minutes over medium high heat. Add the cream, lower the heat to medium, and reduce by half. Add the beef stock and once more reduce by half. Add the lemon juice and any meat juices from the covered plate. Drain the vegetables and stir them into the sauce. Swirl in the butter and season with salt and pepper to taste. Place the meat on plates, topped with the sauce. The sirloin may be garnished with a swirled piping of sour cream, pressed through a pastry bag.
Serves 6.