Heat oven to 400ºF. 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.