Wash the spinach and strip the leaves from the stalks. Place in a large saucepan with a little salted water, bring to the boil, and cook for 5 minutes. Drain and squeeze the spinach and spread on paper towels to dry completely.
Chop the spinach finely. Place in a medium bowl with the ricotta, and a pinch each of nutmeg and pepper, and mix thoroughly. Mix in the eggs, 1/3 cup of the grated Parmesan and the flour. Refrigerate the mixture for a few hours or overnight.
Dust a chopping board with flour, then roll out portions of the chilled gnocchi mixture to form cylindrical shapes the size of corks.
Bring a large saucepan of salted water to the boil, then lower the heat to a gentle simmer. Carefully drop the gnocchi into the water in batches; they will rise to the surface when cooked – about 5 minutes. Lift the gnocchi from the water and drain well. Keep warm.
Melt the butter in a small frying pan. Bruise the sage leaves and add to the pan. Heat until the butter bubbles, pour the sauce over the gnocchi and sprinkle with the remaining Parmesan cheese.