In a large kettle, saute onion in butter until tender.
Add flour, salt and pepper; stir to make a smooth paste.
Gradually add water, stirring constantly.
Bring to a boil; cook and stir for 1 minute.
Add the mushrooms, celery, potatoes and carrots.
Reduce heat; cover and simmer for 30 minutes or until vegetables are tender.
Add cream and Parmesan cheese; heat through.
