Dust the beef with flour and remove any excess. In a large pot over medium heat, sear the meat on all sides in oil. Add the onion and pour in the wine.
Let the liquid evaporate, then cover the meat with water (or broth, according to taste), reduce the heat to low, and cook for about 1 hour, or until the meat is very tender. If needed, add more water as it cooks. Finally, taste and season with salt and pepper before removing from heat.
Meanwhile, bring a pot of salted water to a boil and add the cabbage. Blanch it for a few minutes, then drain and let it cool.
Remove the meat from the cooking liquid (you can save the liquid as a sauce) and finely chop it together with the cabbage using a knife or meat grinder. Add the grated cheese and season with salt and pepper to taste. Mix well, then cover with plastic wrap and let it rest while you prepare the pasta dough.
On a work surface, make a mound with the flour and create a well in the center. Add the eggs and salt into the well. Using a fork, beat the eggs while gradually incorporating the flour until a dough forms.
Knead the dough for about 10 minutes until smooth and elastic. Wrap it in plastic wrap and let it rest for 30 minutes.
Divide the dough into three parts and roll them out one by one using a pasta machine, starting at the widest setting and gradually working down to the thinnest. Lay the pasta sheets on a floured work surface and cut them into long rectangles.
On the lower half of each pasta sheet, place small mounds of filling using a teaspoon, leaving about 1/2 cm of space between them. Carefully fold the top part of the dough over the filling and use a pastry wheel to cut small rectangles. Press the edges firmly to seal and gently pinch each piece to create the characteristic shape of agnolotti del plin.
Bring a large pot of salted water to a rolling boil. Meanwhile, melt the butter in a pan over medium heat for the sauce. Alternatively, strain the remaining meat juices through a fine sieve and use them as a sauce.
Cook the pasta parcels in boiling water until they float to the surface—about 2 minutes. Drain and transfer to the pan with the butter sauce (skip this step if using the meat juices instead). Sauté for 30 seconds to coat evenly in the butter.
Enjoy your meal!