Cook spaghetti noodles in a large pot of salted water according to package instructions until al dente. Drain and set aside.

Preheat the oven to 350°F (175°C).

In a large skillet, combine Italian sausage, ground beef, and chopped onion. Cook until the meats are no longer pink and the onion is tender. Drain off any excess oil.

Add marinara sauce to the skillet and stir to combine. Set aside.

In a medium bowl, beat together ricotta cheese, cream cheese, and egg until well combined.

Spoon 1 cup of the meat sauce into the bottom of a 9x13-inch baking dish. Layer half of the spaghetti noodles, half of the meat mixture, half of the ricotta mixture, and half of the mozzarella and parmesan cheeses. Repeat the layering for the remaining half of the ingredients.

Bake for 30 minutes until the cheese is fully melted and the sauce is bubbling up around the sides of the dish. If desired, broil for 2-3 minutes to brown the top of the cheese.

Remove from the oven and let it cool for about 15 minutes before serving. Optionally, garnish with freshly chopped parsley before serving.