Melt margarine in a large saucepan over medium heat; add mushrooms and onion, and saute 7 minutes or until liquid evaporates.
Stir in all-purpose flour. Gradually add chicken broth, skim milk, and light process cream cheese product; bring to a boil, and cook 5 minutes, stirring constantly. Remove from heat; stir in 2 tablespoons Parmesan cheese and next 5 ingredients. Set aside.
Break spaghetti into 4-inch pieces. Cook according to package directions, omitting salt and fat. Drain well.
Stir spaghetti and chicken into mushroom mixture. Spoon mixture into a deep 3-quart casserole coated with cooking spray. Sprinkle with remaining 2 Tablespoons Parmesan cheese.
Cover casserole and bake at 350 degrees F for 20 minutes. Bake, uncovered, 10 minutes. Let stand for 5 minutes before serving. Yield: 6 servings (about 339 calories per 1 1/3 cup serving). 7 points per serving.