Cook noodles according to package directions, omitting salt and fat. Drain and set aside.
Combine onion, chicken broth, and garlic in a large saucepan; stir well. Cook over medium heat, stirring constantly, until onion is tender. Add noodles, spinach, and mushrooms; stir well.
Combine cheese and next 7 ingredients in a medium bowl; stir well. Add cheese mixture to noodle mixture, stirring well. Spoon into an 11- x 7- x 1 1/2-inch baking dish coated with cooking spray. Cover and bake at 350° for 30 minutes or until thoroughly heated.