In a large saucepan, melt the butter over medium-high heat. Add the mushrooms, celery and onion. Cook and stir until everything is tender and golden, four to six minutes.
Stir in the flour until everything is blended and cook for two to three minutes, stirring constantly.
Gradually whisk in the vegetable stock. Bring the mixture to a boil, then reduce the heat, cover and let it simmer for 10 minutes.
Add the chopped spinach and cook until it wilts into the soup, two to four minutes.
Gradually stir in the half-and-half, sour cream, salt and pepper, and heat through, but do not allow the mixture to boil.
Sprinkle with fresh parsley, and serve with slices of baguette, if desired.
