Cook the spinach, squeeze very dry and chop finely. If using frozen spinach, thaw, squeeze dry and chop. Mix the ricotta in a bowl with the spinach, eggs, 80g of the parmesan, nutmeg and season with salt and pepper. Sprinkle your hands with flour and roll the mixture into small balls and sprinkle them with flour. Bring salted water to the boil in a shallow pan and drop in the gnocchi a few at a time, and as soon as they rise to the surface, remove with a slotted spoon, draining well.
Place on a warm serving dish and keep warm until all the gnocchi are cooked.
Meanwhile melt the butter in a small pan and add the sage leaves. Sprinkle the gnocchi with the remaining cheese, the butter and the sage.