Cook lasagna noodles to desired doneness as directed on package. Drain; place in cold water.
Meanwhile, heat oven to 350°F. In large saucepan, combine milk, Italian seasoning and garlic salt. Bring to a boil. In small bowl, combine broth and cornstarch; blend well. Slowly add cornstarch mixture to hot milk mixture, stirring constantly. Cook over medium heat for 5 minutes or until bubbly and thickened, stirring constantly. Stir in Parmesan cheese.
Drain noodles. Spread 1/2 cup white sauce in bottom of ungreased 13x9-inch (3-quart) baking dish. Top with 3 noodles. In medium bowl, combine 1 cup white sauce, spinach, onions and half of the mozzarella cheese; spread evenly over noodles. Arrange 3 noodles over spinach mixture.
In same medium bowl, combine 1 cup white sauce, mushrooms and chicken; spread over noodles. Arrange 3 noodles over chicken mixture. Top with remaining white sauce. Cover tightly with foil.
Bake at 350°F. for 50 to 55 minutes or until hot and bubbly. Uncover; sprinkle with remaining half of mozzarella cheese. Let stand 15 minutes. If desired, garnish with fresh Italian parsley.