Preheat your oven to 350°F (175°C). Grease a 9x13-inch casserole dish and set aside.

Heat olive oil in a large skillet over medium heat. Add the diced onion and sauté until the onion is translucent, about 3-4 minutes. Stir in the minced garlic and cook until fragrant, about 1 minute.

Add the ground beef and sweet Italian sausage to the skillet. Cook, breaking it up with a spoon, until browned and cooked through, about 6-8 minutes. Drain any excess grease if needed.

Stir in the tomato paste, half of the salt, half of the black pepper, 1/2 teaspoon of Italian seasoning, and crushed red pepper flakes. Cook for 1-2 minutes, then add the crushed tomatoes, 1/4 cup of water, and chopped parsley. Bring the mixture to a simmer and let it cook for about 10 minutes, stirring occasionally.

While the sauce simmers, cook the mostaccioli pasta in salted water according to the package instructions for al dente. Drain and set aside.

In a medium bowl, mix together the ricotta cheese, egg, grated Pecorino Romano, the remaining 1/2 teaspoon of Italian seasoning, 1/2 teaspoon of salt, and 1/4 teaspoon of black pepper until smooth.

Combine the cooked pasta with the sauce mixture, stirring until evenly coated. Spread half of the pasta and sauce mixture into the prepared baking dish. Spread the ricotta mixture evenly over the pasta layer, then top with the remaining pasta and sauce.

Sprinkle the shredded mozzarella cheese and additional Pecorino Romano over the top. Cover the dish with foil and bake for 20 minutes. Remove the foil and bake for another 10 minutes, or until the cheese is melted and bubbly.

Garnish with additional fresh parsley before serving. Enjoy!