For the Braised Oxtail

1. Preheat oven to 300°F (149°C).
2. In a Dutch oven or large pot, heat olive oil over medium-high heat. Season the oxtail with sea salt and freshly ground black pepper. Add the oxtail to the pot and sear until browned on all sides. This may require two batches depending on the size of your pot.
3. Once browned, remove the oxtails and set aside. In the same pot, add carrots, celery, onions, and garlic. Add additional olive oil if necessary. Sauté until the vegetables are browned.
4. Pour in the red wine and deglaze the pot, scraping up the caramelized bits from the bottom.
5. Once the wine has reduced by a third, add the beef broth, tomato paste, parsley, rosemary, and bay leaves.
6. Return the oxtail to the pot and bring the mixture to a boil. Cover the pot and transfer it to the oven. Braise for 3 to 3 1/2 hours, rotating the oxtail occasionally. The meat should be tender and almost falling off the bones.
7. Remove the pot from the oven and transfer the oxtail to a dish. Strain the braising liquid and return it to the pot. Reduce on medium heat until slightly thickened.

For the Agnolotti Filling

1. In a medium saucepan, heat olive oil over medium heat. Add onions and cook until they turn a deep brown, which should take about 12 minutes.
2. Stir in garlic, sugar, and vinegar. Cook for another 2 minutes, stirring continuously.
3. Add the chopped braised oxtail and half a cup of chicken stock. Let it simmer until the mixture reduces and becomes sticky.
4. Pour in the remaining chicken stock and continue cooking until the mixture thickens.
5. Off the heat, stir in bone marrow (if using), parsley, and salt. Once the marrow melts into the mixture, transfer it to a food processor.
6. While hot, puree the oxtail mixture until smooth. Incorporate the Parmigiano cheese and adjust seasoning if necessary.
7. Allow the filling to cool to room temperature, then transfer to a piping bag.

For the Ravioli Dough

1. On a clean work surface, combine flour and salt, forming a mound with a well in the center.
2. In the well, gradually add the eggs, yolks, and olive oil. Gently whisk the eggs without breaking the walls of the flour.
3. Incorporate the flour gradually, eventually kneading with your hands to form a stiff dough. Spritz with water if too dry.
4. Knead the dough for about 10-15 minutes until its smooth and elastic. Wrap in plastic and let it rest for 30 minutes.

Forming the Agnolotti

1. Roll out the rested dough thinly.
2. On a floured surface, cut the dough into two long strips (around 3 inches wide). Pipe the oxtail filling along one strip, leaving a small space between each portion.
3. Mist the edges with water and fold the dough over the filling, pressing out air bubbles and sealing the edges.
4. Using a pasta wheel, trim the long edge for a neat finish and then cut between each filling to create individual agnolotti.

Cooking the Agnolotti

1. Bring a large pot of salted water to a boil. Gently drop the agnolotti into the boiling water.
2. Cook for 3-4 minutes or until they rise to the surface and are al dente.
3. Drain and serve with your preferred sauce or simply drizzled with some olive oil and a sprinkle of Parmigiano-Reggiano.