Cook pasta according to package directions; drain and place in a large bowl. Add the mushroom, pepperoni, green pepper and onion. Stir in the spaghetti sauce, tomato sauce, tomato paste, and cheddar cheese; mix well.
Transfer to a greased 3-quart baking dish. Cover and bake at 350F for 50 minutes. Uncover; sprinkle with mozzarella cheese. Bake 10 minutes longer or until cheese is melted.