About 3 cups freshly grated Parmesan cheese (3/4 pound)
1/2 teaspoon freshly grated nutmeg
Salt and freshly ground pepper
5 extra-large egg yolks
1 cup all-purpose flour
1 stick (4 ounces) unsalted butter
Fresh sage leaves, for garnish
Spoon the ricotta into a large coffee filter set in a strainer and drain for 1 hour.
Bring a large pot of salted water to a boil. Add the spinach and cook for 10 minutes, then drain in a colander and cool under cold running water. Drain the spinach thoroughly. Working with one handful at a time, squeeze the spinach until very dry. Finely chop the spinach.
In a large bowl, combine the drained ricotta with the spinach, 2 cups of the Parmesan and the nutmeg. Season generously with salt and pepper, add the egg yolks and stir until evenly combined.
Bring a large pot of salted water to a boil. Spread the flour on a plate. Form level tablespoons of the spinach-ricotta mixture into balls. Roll the balls lightly in the flour until coated. Arrange the balls on a lightly floured baking sheet.
Melt the butter in a medium skillet. Pour the butter into a large, warmed baking dish and keep warm near the stove.
Gently drop one-third of the balls into the pot and cook just until they rise to the surface. Using a wire skimmer or slotted spoon, transfer the ravioli to the baking dish in a single layer. Return the water to a boil and cook the remaining balls in 2 batches. Sprinkle the ravioli nudi with as much of the remaining Parmesan as desired, garnish with sage leaves and serve immediately.