Combine spinach, onions and stock in a saucepan, and turn heat to medium high.
Bring to a boil, and lower heat so mixture barely bubbles.
Cook, stirring occasionally, until spinach is very tender, about 10 minutes.
Turn off heat, add nutmeg and a sprinkling of salt and pepper
Let cool at least a few minutes.
Pour soup into a blender, in batches if necessary, and carefully purée.
Return to pan, add cream, and, over medium-low heat, reheat gently, stirring occasionally.
When soup is hot, adjust seasoning, and serve.