Cook spaghetti to desired doneness as directed on package. Drain.
Meanwhile, heat oven to 350°F. In large saucepan, cook onion in margarine over medium heat until tender. Stir in flour, salt, sage and pepper; cook until bubbly. Stir in broth, milk and mushrooms. Cook and stir until mixture boils and is slightly thickened.
Stir in cooked spaghetti and all remaining ingredients except Swiss cheese. Spoon into ungreased 12x8-inch (2-quart) baking dish; sprinkle with Swiss cheese.
Bake at 350°F. for 20 to 30 minutes until hot and bubbly. If desired, sprinkle with additional chopped fresh parsley.