1. Cook the spinach in boiling water for 5 minutes. Drain and squeeze out the excess water.
2. In a large sauté pan, melt the butter over medium heat. Sauté the onion until golden. Add the spinach, stir, and cook 3 minutes longer. Transfer to a mixing bowl.
3. Add the ricotta, nutmeg, and egg yolk and mix well. Season with salt and pepper to taste and stir in 3/4 cup of the Parmesan. Set filling aside.
4. To form ravioli, roll out the Basic Pasta dough until very thin (No. 6 on the pasta machine). Place 1 sheet of rolled dough on a ravioli form, being careful to cover all sides evenly. Press on the top mold of the dough form to make hollows for the filling. Fill each hollow with 1 teaspoon of the filling. Brush the edges of the ravioli with the beaten egg.
5. Place another sheet of rolled pasta over the filling to cover. Run a rolling pin over the top of the rack to seal and separate the ravioli. Transfer to a baking sheet dusted with cornmeal. (If making in advance, turn the ravioli every hour to dry evenly or store in zip-top freezer bags and store in the freezer.)
6. To cook, bring a large pot of water to a boil. Add the ravioli, a handful at a time, and cook until they rise to the top.
7. Meanwhile, place 1 cup of the Tomato Sauce in the bottom of a warmed large mixing bowl. With a slotted spoon, remove the cooked ravioli and transfer to the bowl. Top the ravioli with butter, 1/2 cup of the basil, and the remaining Tomato Sauce. Gently toss to coat the ravioli evenly.
8. To serve, spoon the ravioli into heated pasta bowls, sprinkle with the remaining basil and 1 teaspoon Parmesan cheese. Pass the remaining Parmesan at the table.
Note: Ravioli can be made in advance, wrapped in plastic after dry, and frozen for up to 1 month.