Cook the spinach with a little salt–water is not necessary–and strain and squeeze it dry with your hands. Chop the spinach finely, add to a pan with melted butter and mix in well. Leave the spinach to cool. When cooled add the sieved ricotta and 100 g/4 oz of grated Parmesan, the eggs and a pinch of nutmeg. If the mixture is too soft, add 1 tablespoon of flour. Lightly flour a cloth, make the gnocchi by forming them into balls the size of a large marble, and dent the centre slightly with a thumb. Keep the hands lightly floured. As the gnocchi are prepared, leave them on the floured cloth.
Bring a large saucepan of lightly salted water to the boil. Boil the gnocchi for a few minutes, until they rise to the surface of the water.
Butter a gratin dish or 4 single ones, make one layer of the gnocchi and cover with tomato sauce. Sprinkle with the rest of the grated Parmesan cheese and pass under the grill until the cheese is coloured.
These gnocchi can also be served with melted butter infused with sage leaves.