1Heat 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.
2Mix baking mix, egg and milk until blended. Spread evenly over beef mixture. Top with bell pepper rings.
3Bake 18 to 22 minutes or until topping is light golden brown. Let stand 5 minutes before cutting.