Preheat the oven to 350°F (175°C). Heat the olive oil in a large skillet over medium heat. Add the chopped onion and cook until softened and lightly browned. Stir in the garlic and cook for 30 seconds until fragrant.

Add the Italian sausage and ground beef to the skillet. Cook, breaking apart with a spoon, until fully browned with no pink remaining. Season with salt, pepper, and 1½ teaspoons of the Italian seasoning. Remove the meat mixture from the skillet and set aside, leaving the drippings in the pan.

Add the diced tomatoes with their juice and the tomato sauce to the skillet. Stir in the remaining Italian seasoning, red pepper flakes (if using), and additional salt and pepper as needed. Mix well, scraping up any browned bits from the pan. Remove 1 cup of the sauce and set aside for later layering. Cover the skillet and let the remaining sauce simmer gently for 20 minutes.

While the sauce simmers, bring a large pot of salted water to a boil. Cook the rigatoni until just shy of al dente, 1–2 minutes less than the package directions. Drain well.

In a medium mixing bowl, combine the ricotta cheese, egg, half of the mozzarella cheese, parmesan cheese, and 1 teaspoon Italian seasoning. Mix until well blended.

Place the drained rigatoni in a large bowl. Add the cheese mixture and toss until evenly coated. Pour in most of the simmered sauce (reserving the cup set aside earlier) and gently mix until the pasta is well covered.

Lightly grease a 9×13-inch baking dish. Spread half of the pasta mixture evenly across the bottom. Layer all of the cooked meat mixture over the pasta. Drizzle with half of the reserved sauce. Add the remaining pasta mixture, top with the remaining reserved sauce, and sprinkle evenly with the remaining mozzarella cheese.

Transfer the dish to the oven and bake for 45 minutes, or until the cheese is melted and golden and the casserole is bubbling. Let rest for 5 minutes before serving. Garnish with dried basil, if desired.