Heat oven to 400º. Grease rectangular baking dish, 11x7x1 1/2 inches. Cook beef
and onion in 10-inch skillet over medium heat, stirring occasionally, until beef is
brown; drain. Stir in gravy mix (dry), water and mushrooms. Cook 1 to 2 minutes,
stirring constantly, until thickened; remove from heat. Stir in sour cream. Spoon
mixture into baking dish.