Mix together the spinach, ricotta, butter, Parmigiano, and eggs with a rubber spatula in a large bowl. Season with nutmeg, salt, and pepper. Sift the flour through a sieve into the spinach mixture. With a spatula, mix it just enough to incorporate the flour. Overmixing the dough will make the gnocchi heavy and tough. The dough will be soft and a little sticky — that’s fine. Refrigerate in a covered container for a few hours or, better still, overnight.
About 20 minutes before serving, melt the butter in a small pan with the sage leaves over medium heat. Turn off heat, cover, and keep warm.
To form and cook the gnocchi, fill a wide pan with water to a depth of about 3 inches. Season with salt, and bring to a gentle simmer over medium heat. Adjust the heat to keep the water barely simmering. Have ready the chilled gnocchi dough, 2 teaspoons, and 1 cup cold water. Dip the spoons in the water, then scoop up some dough with one spoon. Use the other spoon to shape the gnoccho into a quenelle. Hold the spoon in the simmering water for a second and the gnoccho will slide off to the bottom of the pan. Cook 6–10 gnocchi at a time. When they float to the surface, cook them for about 3 minutes.
Divide the gnocchi between flat soup bowls and spoon the warm sage butter on top. Season with salt and pepper and a shower of Parmigiano.