Prepare the Pasta Dough: On a clean surface, make a mound with the flour and create a well in the center. Crack the eggs into the well, add the olive oil and salt, and gradually incorporate the flour into the eggs using a fork. Knead the dough until smooth and elastic, about 10 minutes. Wrap it in plastic wrap and let it rest for at least 30 minutes.

Make the Filling: In a bowl, combine the ricotta, spinach, Parmesan cheese, egg yolk, salt, pepper, and nutmeg (if using). Mix until well combined.

Assemble the Ravioli: Divide the dough into four pieces. Working with one piece at a time, roll out the dough into a thin sheet using a pasta machine or a rolling pin. Place teaspoons of filling an inch apart over half of the pasta sheet. Fold the other half over the filling and press down around the edges to seal, removing any air pockets. Cut into individual ravioli with a pastry cutter or knife. Repeat with the remaining dough and filling.

Cook the Ravioli: Bring a large pot of salted water to a boil. Cook the ravioli in batches until they float to the surface, about 3-4 minutes. Remove with a slotted spoon and drain.

Prepare the Sauce: Melt the butter in a large skillet over medium heat. Add the sage leaves and cook until the butter is slightly browned and aromatic. Toss the cooked ravioli in the sage butter, coating them gently.

Serve: Plate the ravioli and drizzle with the sage butter sauce. Sprinkle with grated Parmesan cheese before serving.