Sauté onion in margarine in a dutch oven until tender. Add mushrooms, potatoes, celery, salt, and pepper; cover and simmer 15 to 20 minutes or until tender.
Combine flour and water; mix until smooth and stir into vegetable mixture. Add chicken stock and simmer 10 minutes.
Stir in milk and be careful not to boil mixture. Cook over low heat. Just before serving, sprinkle with parmesan cheese.
Servings: 6 to 8