Preheat the oven to 375°F (190°C). Lightly grease a 9×13-inch baking dish and set it aside.

Cook the spaghetti in a large pot of salted boiling water until al dente, according to package instructions, then drain and set aside.

Meanwhile, heat the olive oil in a large skillet over medium heat. Add the red onion and garlic and cook for 3–4 minutes until softened.

Add the ground beef and Italian sausage to the pan. Break it up with a spoon and cook for 8-10 minutes until browned and fully cooked.

Drain any excess grease, then stir in the Italian seasoning, salt, pepper, and half of the marinara sauce. Let it simmer on low while you prepare the other components.

In a large mixing bowl, stir together the ricotta, cream cheese, sour cream, and 1 cup of the shredded mozzarella until smooth and creamy.

Toss the cooked spaghetti with the remaining half of the marinara sauce until evenly coated.

To assemble the casserole, dot the bottom of the prepared baking dish with half the butter, then spread half of the sauced spaghetti over it. Spoon half of the cheese mixture on top and spread it out as evenly as you can.

Layer the remaining spaghetti on top, followed by the rest of the cheese mixture. Dot the top with the remaining butter, scattered evenly.

Spoon the meat sauce over everything and spread it out evenly. Sprinkle the remaining 1 cup of shredded mozzarella over the top.

Cover loosely with foil and bake for 30 minutes. Remove the foil and bake for another 10–15 minutes, or until bubbly and golden on top.

Let the casserole rest for 10 minutes before slicing. Garnish with freshly chopped parsley and serve warm.