1. Preheat your oven to 350°F (175°C). Lightly grease a 9×13-inch baking dish and set aside.

2. Bring a large pot of salted water to a boil. Add the spaghetti and cook until just al dente, 1–2 minutes less than the package directions. Before draining, scoop out about 1/2 cup of the starchy pasta water and reserve. Drain the pasta well.

3. While the pasta cooks, heat the olive oil in a large skillet over medium-high heat. Add the diced onion and cook for 3–4 minutes until softened and translucent.

4. Stir in the minced garlic and cook for 30 seconds, just until fragrant.

5. Add the ground beef to the skillet. Cook, breaking it up with a spoon, for 6–8 minutes until browned and no longer pink. Drain any excess grease.

6. Pour in the tomato pasta sauce. Stir in the Italian seasoning, red pepper flakes if using, salt, and black pepper. Reduce the heat to low and let the sauce simmer for 5–7 minutes while you prepare the cream cheese mixture.

7. In a medium mixing bowl, add the softened cream cheese, sour cream, and grated Parmesan. Beat or stir until smooth and creamy. If the mixture seems very thick, add 1–2 tablespoons of the reserved hot pasta water and mix again until it becomes silky and spreadable.

8. Place the drained hot spaghetti in a large bowl. Spoon the cream cheese mixture over the pasta and toss until every strand is evenly coated. Add a splash more of the reserved pasta water if needed to help the mixture cling to the noodles.

9. Spread half of the cream-cheese-coated spaghetti into the prepared baking dish in an even layer.

10. Spoon half of the meat sauce over the spaghetti and spread it gently to cover. Sprinkle with 1 cup of shredded mozzarella and 1/2 cup of shredded cheddar.

11. Repeat the layers with the remaining spaghetti, the rest of the meat sauce, and the remaining 1 cup mozzarella and 1/2 cup cheddar, creating a second cheesy layer on top.

12. Cover the baking dish loosely with foil, making sure it doesn’t touch the cheese. Bake for 20 minutes.

13. Remove the foil and continue baking for another 10–15 minutes, or until the casserole is hot, bubbling around the edges, and the cheese is melted and lightly golden.

14. Take the casserole out of the oven and let it rest for 10 minutes to set the layers.

15. Garnish with chopped fresh parsley or green onions. Slice or scoop into portions and serve warm.