Cut steak into 1/8-inch strips and cut the strips into 1-inch lengths.
Heat oil and butter in a large skillet over medium heat. quickly brown the meat, stirring frequently. Add the onion and mushrooms, if using. Cook, stirring, until vegetables are tender and lightly browned. Sprinkle with the flour and stir to blend into the mixture. Add the sour cream and hot water. Cook, stirring, until thickened. Do not boil.
Serve hot with hot cooked noodles or potatoes.
Calories per serving: 434