Wash the spinach and remove the stems. In a large pan with the lid on, steam the leaves with a little salt in the water that clings to them, turning them over, until they crumple. Strain and squeeze every bit of water out with your hands: this is all-important and is the secret of success (otherwise the dumplings would fall apart). Finely chop the leaves.
Mash the ricotta and stir in the eggs, half the parmesan, salt, pepper, nutmeg and spinach. Work very well, shape into balls the size of a walnut and roll in flour.
Fill a large saucepan halfway with water, bring to the boil and very carefully drop in the dumplings. Keep the water barely simmering until the rise to the surface - they do so very quickly.
Lift them out very carefully with a slotted spoon and serve very hot with melted butter and the remaining parmesan.