Heat butter in a large pot over moderate heat and sauté onion until light golden brown. Add spinach and stir until spinach is just tender – about 5 minutes. Transfer spinach mixture and a little of the stock to a blender and process until smooth. Return spinach mixture to pot and add remaining stock. Season with nutmeg, salt and pepper then bring to a simmer. Stir in cream right before serving.
Source: Grouprecipes.com