Make the Bolognese Ragu: In a large pot or Dutch oven, heat the olive oil over medium heat. Sauté the finely chopped onion, carrot, and celery until very soft, about 8-10 minutes. Add minced garlic and cook 1 minute more. Add ground beef and ground pork, increasing heat slightly, and brown thoroughly (about 10 minutes), breaking it apart. Drain excess fat if needed. Pour in the red wine, deglazing the pot, and cook until mostly reduced. Stir in the tomato paste, crushed tomatoes, whole milk, dried oregano, salt, and pepper. Bring to a boil, then reduce heat to low, cover, and simmer for at least 2 hours (up to 3 for best flavor), stirring occasionally.

Cook the Conchiglioni Pasta: While the sauce simmers, bring a large pot of salted water to a rolling boil. Carefully add the jumbo conchiglioni shells, ensuring not to overcrowding. Cook for 10-12 minutes, or until just al dente. Carefully drain the shells, rinse immediately with cool water, and place them in a single layer on a parchment-lined baking sheet to prevent sticking and breakages.

Fill the Pasta Shells: Once the shells are cooled slightly and the Ragu is ready, stir 1 cup of grated Parmesan cheese into the finished Ragu if desired for extra creaminess, or leave it meat-focused. Using a spoon or small cookie scoop, generously fill each conchiglioni shell with the Bolognese mixture. Arrange the stuffed shells snugly in individual oven-safe bowls (like in the photo) or a single-layer in a large casserole dish.

Top and Bake (Optional): Top each stuffed shell with a final layer of more Ragu and a dusting of grated Parmesan. If preparing to serve immediately as is, you can skip baking. For individual portions like in the photo, place the stuffed bowls under the broiler for 1-2 minutes until the cheese melts and browns slightly. For a casserole dish, bake covered with foil at 375°F (190°C) for 15 minutes, then uncover and bake 10 minutes more until bubbly.

Final Garnish and Serving: Allow the shells to cool for 2-3 minutes before serving. Garnish generously with more freshly grated Parmesan cheese and finely chopped fresh parsley. Serve immediately with a fork.