This delicious tortelloni recipe is courtesy of Marcella Hazan and can be found in her cookbook, "Essentials of Classic Italian Cooking."
Remove Swiss chard leaves from stalks; discard stalks. Place Swiss chard leaves in a large bowl and fill with enough water to cover; let stand for a minute. Remove leaves from bowl and drain water; repeat process until Swiss chard is clean and no soil remains at the bottom of bowl.
Transfer wet leaves to a large saucepan set over medium heat. Season with salt and cook, covered, until tender, about 12 minutes. Drain and let cool slightly. Gently squeeze out excess moisture from Swiss chard and very finely chop; set aside.
Place onion and butter in a small skillet over medium heat and cook, stirring, until onion is browned. Add prosciutto and cook for 1 minute. Add Swiss chard and cook, stirring, until all the butter has been absorbed, 2 to 3 minutes.
Transfer Swiss chard mixture to a large bowl and add ricotta, egg yolk, Parmigiano-Reggiano, and nutmeg; season with salt and mix with a fork until well combined; set aside.
Set the rollers of a hand-crank or electric pasta machine at their widest opening. Unwrap one piece of dough; flatten with the palms of your hands and run it once through the machine. Fold the dough in thirds like a letter and run through the machine again. Repeat this step two or three times, until the dough is smooth and elastic.
Now change the rollers of the pasta machine to the next decreasing setting, and roll out the dough once without folding. Keep rolling the sheet through the machine on decreasing settings until you have rolled it through the last (thinnest) setting.
Lay a dry, clean towel flat on work surface and set aside. Lay pasta flat on work surface. Place about 1 teaspoon stuffing, 2 inches apart, down the center of the pasta. Fold one long side over the stuffing to cover and join edges. Using a fluted pastry wheel, trim edges and end of pasta. Using the same wheel, cut between every mound of stuffing to seal and separate into squares. Transfer squares to towel, making sure they do not touch. Repeat entire process with remaining dough and filling.
Bring a large pot of water to a boil. Add salt and 1 tablespoon olive oil. Add tortelloni, and cook until tender, about 6 minutes. Using a slotted spoon, gently transfer tortelloni to a colander to drain. Transfer tortelloni to a large platter. Top with sauce and sprinkled Parmigiano-Reggiano; serve immediately.