Step 1: Begin by preparing your aromatic base. In a large, heavy-bottomed pot or Dutch oven, heat 2 tablespoons of olive oil over medium-high heat. Add 1.5 lbs ground beef and cook, breaking it up with a spoon, until deeply browned. Drain off any excess fat and set the meat aside.
Step 2: Reduce the heat to medium, add another tablespoon of olive oil to the same pot. Add 1 chopped yellow onion, 2 chopped carrots, and 2 chopped celery stalks. Sauté for 8-10 minutes until softened and translucent, scraping up any browned bits from the bottom of the pot.
Step 3: Stir in 4 cloves of minced garlic and cook for 1 minute until fragrant. Pour in 1/2 cup of dry red wine and scrape the bottom of the pot to deglaze, allowing the alcohol to cook off for 2-3 minutes.
Step 4: Return the browned ground meat to the pot. Add a 28-ounce can of crushed tomatoes, a 15-ounce can of tomato sauce, 1/4 cup tomato paste, 1 cup beef broth, 2 bay leaves, 1 teaspoon dried oregano, 1/2 teaspoon dried basil, and a pinch of red pepper flakes (optional). Season generously with salt and black pepper.
Step 5: Bring the ragu to a simmer, then reduce the heat to low, cover, and let it cook for at least 2-3 hours, stirring occasionally. Remove the bay leaves before assembling.
Step 6: While the ragu simmers, cook 1 lb of rigatoni pasta according to package directions, but aim for very al dente – about 2-3 minutes less than recommended. Drain well and let it cool slightly. Don't rinse it!
Step 7: Preheat your oven to 375°F (190°C). Lightly grease a 9x13 inch baking dish. Pour about 1 cup of the beef ragu into the bottom of the prepared baking dish and spread it evenly.
Step 8: Now for the fun part: stuffing the rigatoni! Using a small spoon, piping bag, or even your fingers, carefully fill each piece of cooked rigatoni with some of the beef ragu.
Step 9: Arrange the stuffed rigatoni vertically (standing up) in the baking dish over the layer of sauce. Pack them in snugly to create a beautiful, uniform layer.
Step 10: Once all the rigatoni are arranged, spoon the remaining beef ragu over the top, making sure to cover all the pasta.
Step 11: Generously sprinkle 2 cups of shredded low-moisture mozzarella cheese over the entire casserole. For extra depth of flavor, you can also add 1/2 cup of grated Parmesan cheese.
Step 12: Cover the baking dish tightly with aluminum foil and bake for 30 minutes. This allows the pasta to finish cooking and the flavors to meld without the cheese burning.
Step 13: Remove the foil and continue to bake for another 15-20 minutes, or until the cheese is bubbly, golden brown, and slightly crispy in spots. Let the baked rigatoni stuffed beef ragu mozzarella recipe rest for 10-15 minutes before serving.