Wash and trim the celery, and cut it in pieces. Clean the mushrooms and cut two-thirds of them into quarters. Cut the remaining third into slices.
Sauté the celery in a large skillet with 1 tablespoon of the butter. When quite soft, put it into a blender with half the chicken stock, and purée. Put the purée into a soup pot.
Sauté the quartered mushrooms in two tablespoons of the butter until tender and lightly browned. Season them with the savory, and some pepper. Purée them with the remaining chicken stock, and add them to the soup pot. Season with salt, according to the saltiness of the chicken stock. Add the sherry and cream with the starch well mixed into it, and heat through. Don''t let it boil, but heat it long enough to thicken slightly.
Meanwhile, sauté the remaining mushrooms in the last of the butter. Sprinkle them over the soup as it is served.