Preheat oven to 300F.

Heat the olive oil over medium-high heat in a Dutch oven or large pot. Add the oxtail and sear, rotating to brown all four sides. You can do this in two batches if your pot is not large enough to accommodate all the meat at once.

Remove the oxtail and add the carrot, celery, onion, and garlic. If the mixture seems dry, add more olive oil, and then sauté the vegetables until they are brown. Add the red wine. Using a wooden spoon, dislodge the caramelized sugars from the bottom of the pan. Stir and continue cooking until the liquid has reduced by a third.

Add the beef broth, tomato paste, parsley, rosemary, and bay leaves. Return the oxtail to the pot, cover, and bring to a boil.

Place the covered pot in the oven and cook for 3-3 ½ hours, turning once or twice. The oxtail is done when the meat is barely clinging to the bones. You should be able to pull them straight off.

Remove the oxtail from the pot and keep them warm in the oven. Skim the fat from the liquid, strain the remaining liquid, and pour back in a saucepan. Over medium heat, let the liquid reduce until it begins to thicken.

In a medium saucepan, heat the olive oil over medium heat. Add the onion and cook until deep brown, about 12 minutes. Add the garlic and cook another minute. Stir in the sugar and vinegar. Cook, stirring continuously, for 2 minutes. Add the chopped oxtail and 1/2 cup of chicken stock. Simmer and cook, stirring occasionally until the sauce has reduced and become sticky, about 5 minutes.

Add the remaining 1/2 cup of chicken stock and cook until the filling comes together and is thick and deeply rich. Remove the pan off the heat and add the bone marrow (if using), parsley, and 1 teaspoon of salt. Stir until the bone marrow has melted into the sauce.

Transfer the mixture to a food processor while still hot and puree until smooth, about 1 minute. Add the Parmigiano cheese and puree. Taste and re-season with salt and vinegar as needed.

Let the mixture cool to room temperature. Transfer to a piping bag.

See instructions above (in the body of the post) for forming the agnolotti.

Mix the flour and salt. Place the flour on a dry, clean work surface forming a mound. Create a well in the middle with the bottom of a measuring cup.

Slowly add the eggs, egg yolks, and olive oil. With a fork, gently beat the eggs; be careful not to disturb the walls of the flour. Slowly begin to incorporate the flour walls into the egg mixture. Continue mixing the flour with the eggs until you have a solid mass.

At this point, with your hands, start folding and forming the dough, incorporating the rest of the flour. Use a spray bottle to spritz and moisten the dough until you have a stiff, solid mass (removing any dry clumps of flour).

Knead the dough. Drive the heel of your hand into the dough, rotate the dough 45 degrees and repeat for 10-15 minutes, until the dough is firm and bouncy, and has a smooth, silk-like texture. Tightly wrap the dough in plastic and let rest 30 minutes.