In a large saucepan, heat butter over medium-high heat. Add mushrooms, celery and onion; cook and stir 4-6 minutes or until tender. Stir in flour until blended; cook and stir 2-3 minutes or until lightly browned. Gradually whisk in stock. Bring to a boil. Reduce heat; simmer, covered, 10 minutes.
Add spinach; cook and stir 2-4 minutes or until wilted. Gradually stir in cream, sour cream, salt and pepper; heat through (do not allow to boil). Sprinkle with parsley.