1. Preheat your oven to 375°F (190°C). Grease a 9×13-inch baking dish and set aside.
2. Cook the spaghetti according to package instructions until al dente. Drain and set aside.
3. In a large skillet, cook the ground beef over medium heat until browned. Drain any excess fat.
4. Add the diced onion and garlic to the skillet, cooking until the onion is translucent, about 3-4 minutes.
5. Stir in the marinara sauce, diced tomatoes, Italian seasoning, salt, and pepper. Let the mixture simmer for 5-7 minutes, allowing the flavors to meld.
6. In a large mixing bowl, combine the cooked spaghetti with the meat sauce.
7. Spread half of the spaghetti mixture into the prepared baking dish. Top with dollops of ricotta cheese, then sprinkle with half of the mozzarella and Parmesan cheeses.
8. Layer the remaining spaghetti mixture over the cheeses, then top with the rest of the mozzarella and Parmesan.
9. Cover with foil and bake for 20 minutes. Remove the foil and bake for another 10-15 minutes, until the cheese is melted and bubbly.
10. Garnish with chopped parsley and serve hot.