Coarsely crack the black peppercorns. Rub half of the peppers into each side of the steak. Place steak on the unheated rack of a broiler pan. Broil 3 inches from heat for 6 minutes. Turn steak over and broil for 6 to 8 minutes more or until steak is to desired doneness (160 degrees F for medium doneness).
Meanwhile, for sauce, in a saucepan combine mushrooms, green onion, water, and bouillon granules. Cover and cook over medium heat about 5 minutes or until mushrooms are tender. Stir together yogurt, flour, and mustard. Stir yogurt mixture into mushroom mixture. Cook and stir until thickened and bubbly. Then, cook and stir for 1 minute more.