Rinse the spinach. Wilt the spinach in a pot until bright green, about 3 minutes. Let the spinach cool slightly and then squeeze out all of the water. Roll the spinach in a kitchen towel to remove any residual water. Chop finely.
In a big bowl lightly beat the egg yolks. Then add the spinach, egg yolks, cheeses, ½ cup flour, nutmeg, 1 teaspoon salt and pepper.
Mound a generous tablespoon of flour on a cutting board. Set up a parchment lined sheet tray or plate. Using a 1 ounce scoop, with floured hands form the gnudi. Drop it into the flour and quickly roll to form a light coat. Repeat and refrigerate, uncovered until ready to cook.
Bring a large pot of water to boil. Add half of the gnudi and stir gently to prevent them from sticking together. Cook about 5 minutes or until the gnudi rises to the surface.
While the gnudi cooks, melt the butter in a small saucepan over medium high heat. Stir in the chopped sage and the sage leaves. Add 1-½ tablespoons of gnudi cooking water, reduce heat to low and cook for a few more minutes. Season with salt and pepper.
Using a slotted spoon, remove the gnudi from the cooking water, strain off any excess water and transfer to plates. Cover with sage butter and enjoy immediately!
Serves 3-4 (Makes 12 gnudi balls)