Preheat the oven to 350°F (175°C) and grease a 9x13-inch baking dish.

Cook the spaghetti in salted water until just al dente, then drain and set aside.

In a large skillet over medium heat, cook the ground beef, Italian sausage, and diced onion until browned and cooked through. Drain excess grease.

Stir in the garlic and cook for 1 minute, then add the marinara sauce, tomato sauce, Italian seasoning, salt, and pepper. Simmer for 5 minutes.

In a medium bowl, mix the softened cream cheese, ricotta cheese, sour cream, 1 cup of the mozzarella, and the Parmesan until smooth.

Spread half of the cooked spaghetti in the prepared baking dish and drizzle with half of the melted butter.

Spread the cheese mixture evenly over the spaghetti.

Top with the remaining spaghetti and drizzle with the rest of the butter.

Pour the meat sauce evenly over the top, then sprinkle with the remaining 1 cup mozzarella cheese.

Bake for 30 to 35 minutes, until bubbly and hot. Let rest for 10 minutes before serving. Garnish with parsley if desired.