Step 1: Heat olive oil in a large Dutch oven or heavy-bottomed pot over medium-high heat. Add the ground beef and cook, breaking it up with a spoon, until deeply browned and no pink remains. Drain excess fat and set the beef aside.
Step 2: Reduce heat to medium. Add diced onion, carrots, and celery to the pot. Sauté until softened, about 8-10 minutes. Add minced garlic and cook for another minute until fragrant.
Step 3: Pour in the red wine, scraping up any browned bits from the bottom of the pot. Let it simmer until almost completely reduced, about 5 minutes. This deglazing step adds immense flavor.
Step 4: Stir in the crushed tomatoes, tomato paste, beef broth, bay leaves, and dried oregano. Return the browned beef to the pot. Bring to a simmer, then reduce heat to low, cover, and cook for at least 2-3 hours, or up to 4 hours, stirring occasionally, until the meat is very tender and the sauce has thickened. Season with salt and pepper to taste. Remove bay leaves before proceeding.
Step 5: While the ragu simmers, cook the rigatoni according to package directions, but only to about 75% al dente (a minute or two less than recommended). It should still be quite firm. Drain well and immediately rinse with cold water to stop the cooking and prevent sticking. Toss with a drizzle of olive oil.
Step 6: In a large bowl, combine the ricotta cheese, grated Parmesan cheese, chopped fresh parsley, an egg (for binding), a pinch of nutmeg, salt, and black pepper. Mix until well combined.
Step 7: Carefully transfer the ricotta mixture into a piping bag with a large round tip (or a freezer bag with a corner snipped off).
Step 8: Preheat your oven to 375°F (190°C).
Step 9: Spread a thin layer of beef ragu (about 1 cup) on the bottom of a 9x13-inch baking dish.
Step 10: Stand each rigatoni pasta tube upright in the baking dish. You’ll arrange them in rows, packing them in tightly.
Step 11: Pipe the ricotta filling into each rigatoni tube. Fill them generously!
Step 12: Ladle the remaining beef ragu over the stuffed rigatoni, ensuring all the pasta is covered. Gently push down to ensure the sauce gets into the tubes.
Step 13: Sprinkle generously with shredded mozzarella and a final dusting of Parmesan cheese.
Step 14: Cover the baking dish tightly with aluminum foil and bake for 25 minutes.
Step 15: Remove the foil and bake for an additional 15-20 minutes, or until the cheese is bubbly and golden brown. Let the Stuffed Rigatoni with Beef Ragu rest for 10-15 minutes before serving to allow the flavors to meld and the dish to set slightly.