Cook the Pasta: Bring a large pot of heavily salted water to a boil. Add ziti and cook for 2-3 minutes less than the package directions for al dente. The pasta should be very firm. Drain well and set aside.

Brown the Meats: While pasta cooks, heat olive oil in a large Dutch oven over medium-high heat. Add the ground beef and Italian sausage. Cook, breaking up the meat with a spoon, until browned, about 8-10 minutes. Drain excess grease.

Sauté Aromatics: Add the chopped onion to the pot and cook for 5 minutes until softened. Stir in the minced garlic and red pepper flakes, cooking for 1 more minute until fragrant.

Simmer the Sauce: Pour in the crushed tomatoes and add the Italian seasoning. Season generously with salt and pepper. Bring to a simmer, then reduce heat to low, cover, and let it bubble gently for at least 15-20 minutes.

Prepare Ricotta Filling: In a medium bowl, mix the ricotta cheese, 1/2 cup of the Parmesan cheese, the egg, and a pinch of salt and pepper until smooth and well combined.

Assemble the Casserole: Preheat oven to 375°F (190°C). Add the drained pasta to the pot with the meat sauce and stir to coat. Spread a thin layer of sauce on the bottom of a 9x13-inch baking dish. Layer half of the pasta/sauce mixture in the dish. Dollop with half of the ricotta mixture and sprinkle with half of the mozzarella.

Finish Layering: Add the remaining pasta and sauce mixture. Dollop with the remaining ricotta mixture. Top with the remaining mozzarella and the final 1/2 cup of Parmesan cheese.

Bake to Perfection: Cover the dish tightly with aluminum foil and bake for 25 minutes. Remove the foil and bake for another 15-20 minutes, until the cheese is golden brown and bubbly. Let it rest for 10-15 minutes before serving to allow it to set.