In a large pot, heat oil and butter. Add onion and sauté until translucent.

Add mushrooms and sauté about three minutes more.

Add flour and stir until dissolved. Slowly add the milk and water, stirring continuously to prevent lumps from forming.

Season with salt, pepper, and nutmeg, and add celery stalk.

Bring to a boil, then lower heat and simmer about 30 minutes.

Remove celery from pot. Stir in cream and adjust seasoning. If the soup is too thick, add more liquid.