Fit a large pot with a steamer insert. Add enough water to come about 3 inches below bottom of insert, and bring to a simmer. Add spinach, cover, and steam until bright green, 3 to 5 minutes. Drain, and let cool slightly. Press to remove liquid. Roll spinach in a clean kitchen towel or cheesecloth, and squeeze to remove any remaining liquid. Transfer to a food processor, and puree until smooth. Stir together spinach puree, egg yolks, cheeses and nutmeg.
Mound 1-cup flour on a cutting board. Using floured hands, gently shape 1 tablespoon spinach mixture into a small log. Drop it into the flour, and quickly roll to coat lightly. Transfer to a baking sheet that's lightly dusted with semolina.
Bring a large pot of salted water to a boil. Add half the gnudi, and stir gently to prevent them from sticking together. Cook until gnudi rise and remain on the surface, about 5 minutes. Remove to a platter. Repeat with remaining gnudi. Melt butter in a small skillet over medium-high heat. Stir in chopped sage and the sage leaves. Add 1 1/2 tablespoons gnudi cooking water, reduce heat to low, and cook for 5 minutes. Season with salt and pepper and serve immediately with the gnudi.