1. Preheat oven to 400 degrees. Season oxtail with salt, and place in an even layer in a Dutch-oven over medium-high heat, working in batches if necessary. Cook until well browned on all sides, 3 to 5 minutes per side. Remove oxtail from Dutch-oven and set aside. Return Dutch-oven to stove and warm over medium heat; add onion. Cook, stirring, until caramelized, about 10 to 15 minutes. Add 8 garlic cloves and cook, 2 minutes more.

2. Transfer onion mixture and oxtail to a Dutch-oven. Add beef stock, orange zest, and juice of 2 oranges. Place thyme in a piece of cheesecloth; tie with kitchen twine to enclose and add to Dutch-oven. Season with salt and pepper. Cover and transfer to oven. Cook, until meat is falling off the bone, 2 1/2 to 3 hours.

3. Remove meat from Dutch-oven and let cool; discard bones. Strain braising liquid into a small, clean saucepan through a fine-mesh sieve, pressing down on solids; discard solids. Place saucepan over medium heat and cook until reduced by 3/4; set braising liquid aside.

4. Transfer meat to the bowl of a food processor and pulse until finely chopped. Transfer to a large bowl and set aside.

5. In a large skillet, heat 1/4 cup olive oil over medium heat. Add 5 cloves garlic and cook, stirring, until softened; remove from skillet and discard. Add Swiss chard and cook, stirring, until tender. Remove from heat and let cool slightly. Squeeze out any excess liquid from Swiss chard; set aside 3/4 cup and keep warm. Add remaining Swiss chard to the bowl of a food processor; pulse until finely chopped. Add to bowl with oxtail, along with cheese, 3 tablespoons olive oil, 1 egg, and juice of 1 orange; season with salt and pepper and set aside.

6. Sprinkle a baking sheet with semolina flour and 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.

7. Now change the rollers of the pasta machine to the next lowest 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. Transfer pasta to prepared baking sheet and cover with plastic wrap. Repeat process with remaining pieces of dough.

8. Sprinkle a second baking sheet with semolina flour; set aside. Lay one sheet of pasta flat on work surface with the long side facing you. Beat remaining egg and brush over right half of the pasta. Transfer oxtail mixture to a disposable pastry bag and snip the end, or using a spoon, place about 1 tablespoon oxtail mixture, 2 inches apart, down the center of the right side of pasta. Fold the left side over filling. Using the blunt side of a 1 1/2-inch round cutter, gently press around each mound of filling, but do not cut through. Using a 2-inch round cutter, cut out ravioli. Transfer ravioli to prepared baking sheet and cover with plastic wrap. Repeat process with remaining sheets of pasta and oxtail mixture. Set ravioli aside.

9. Bring a large pot of water to a boil. Generously salt water and return to a boil. Meanwhile, transfer reserved braising liquid to a large skillet. Add butter and cook over medium heat until butter is melted and liquid is heated through.

10. Add ravioli to boiling water and cook until just tender, 2 to 3 minutes. Using a slotted spoon, transfer ravioli to butter mixture and stir to coat. Divide reserved Swiss chard leaves evenly between 4 to 6 serving dishes; top with ravioli and sauce. Garnish with shaved Parmesan and serve immediately.