Melt the butter in a very large saucepan. Add the scallions and celery and cook gently for about 5 minutes, until soft.
Pack the spinach into the saucepan, add the vegetable stock and bring to a boil. Reduce heat and simmer, covered, for 15-20 minutes.
Transfer the soup to a blender or food processor and blend until smooth, or pass through a strainer. Return to the saucepan.
Add the Mascarpone cheese to the soup and heat gently, stirring, until smooth and blended. Season with salt and pepper to taste.
6 servings