Place the cooked spinach in a large bowl with the ricotta, eggs and Parmigiano and mix together. Add the flour, nutmeg and salt and mix to thoroughly combine the ingredients.
Bring a large pot of salted water to a boil.
Cover a cutting board or counter top with flour. Use two teaspoons to form small balls with the spinach mixture and roll in the flour. When all the mixture has been made into balls, place several at a time into the boiling water and cook until they begin to float, 2- 3 minutes. Remove with a slotted spoon, and place on a serving dish.
In a small skillet, over low heat, melt the butter with the sage. Pour over the gnudi.