Wash the spinach carefully and remove any yellow or rotten leaves. Pour 2/3glasses of water in a pot to a boil, add salt and blanch the leaves for 2minutes: drain very well and let them cool down the temperature. Squeeze the spinach as wrote above. On a cutting board finely mince the spinach and transfer in a bowl.
In the bowl add also the ricotta cheese, the 4 tbsp of Parmigiano, the egg, ground nutmeg, salt and mix well.
Prepare small oval-shaped meatballs, lightly flour them and align them on a floured plate. Making rissoles with this humid mixture could be difficult, it will sticky on your hands. Here is the solution! Take two tbsp’s. The first one, fill it halfway with what you have mixed in the bowl; the second one use it as a small shovel to free the first tbsp of its load and if the shape is not yet satisfactory, use again the first tbsp in reverse order, until you get the shape of a quenelle.
Melt butter with sage leaves and keep warm.
Bring another large pot of water (be generous this time, as for cookingpasta) to boil and add salt. Drop the Gnudi into the boiling water; drain with a slotted spoon soon as they float to the surface and transfer to a heated serving bowl.
Add the sauce to the Gnudi, toss gently and serve immediately with the remaining Parmigiano Reggiano.