For 4–6 serving:
Wash, trim and boil the spinach in a little salted water, then drain and leave to cool. Squeeze it out well, and chop finely. Mash the ricotta thoroughly in a bowl, and mix it with the spinach, egg yolks, and a few spoonfuls of grated Parmesan. Season with salt, pepper and grated nutmeg.
If the mixture is too stiff, add the egg whites or a drop of warm milk, then use the filling to stuff the pasta squares. Roll up the cannelloni, and lay then in an ovenproof dish with a layer of béchamel in the bottom. Pour over the rest of the sauce, then sprinkle with grated Parmesan and dot with a few flakes of butter. Brown in a preheated oven at 180°C for about half an hour.