How to Make It
Bring a large pot of water to a boil. Add one-third of the spinach and boil just until wilted. Using a slotted spoon, transfer the spinach to a colander and let cool under running water. Repeat with the remaining spinach. Squeeze the spinach as dry as possible.
Finely chop the cooked spinach and transfer it to a large bowl. Add the egg yolks, ricotta, 1 1/2 cups of the Parmesan, 1/2 cup of the flour, the salt, pepper, grated lemon zest and nutmeg. Stir to form a soft dough.
Divide the dough into 8 pieces. On a lightly floured surface, roll each piece of dough into a 1-inch-thick rope. Cut the rope into 1-inch pieces. Using floured hands, roll the pieces into balls and flatten slightly. Transfer the gnocchi to a generously floured rimmed baking sheet.
Bring a large pot of salted water to a boil. Pour half of the melted butter into a large baking dish. Add one-fourth of the gnocchi to the boiling water, gently stir once and let the gnocchi rise to the surface. Simmer the gnocchi until tender and cooked through, 2 to 3 minutes. Using a slotted spoon, transfer the gnocchi to the buttered baking dish. Cover with foil and cook the remaining gnocchi in batches. Reserve 1/2 cup of the cooking water.
In a large, deep skillet, combine the remaining 4 tablespoons of melted butter and 1/2 cup of Parmesan with the reserved gnocchi cooking water and simmer over moderate heat until slightly thickened, about 4 minutes. Pour the sauce over the gnocchi and toss to coat. Sprinkle with additional Parmesan and serve right away.