1. Cook the Spaghetti:
* Bring a large pot of generously salted water to a rolling boil.
* Add the spaghetti and cook according to package directions until al dente (usually about 8-10 minutes). It should still have a slight bite, as it will cook a bit more in the oven.
* Drain the spaghetti well and set aside. You can toss it with a tiny drizzle of olive oil to prevent sticking if it will sit for more than a few minutes.

2. Prepare the Meat and Aromatics (if using meat):
* If using ground beef or Italian sausage, heat a large skillet or Dutch oven over medium-high heat.
* Add the meat and cook, breaking it apart with a spoon, until browned (about 5-7 minutes).
* Drain off any excess fat from the skillet.
* Add the chopped onion to the skillet with the meat. Cook, stirring occasionally, for 3-4 minutes until the onion softens.
* Stir in the minced garlic and cook for another minute until fragrant.
* (If making vegetarian): Heat 1 tablespoon of olive oil in the skillet over medium heat. Add the onion and cook until softened (3-4 minutes). Add the garlic and cook for 1 minute more.

3. Build the Cheesy Tomato Sauce:
* Pour the jar of marinara sauce into the skillet with the meat/aromatics.
* Stir in the cubed cream cheese, Italian seasoning, red pepper flakes (if using), ½ teaspoon salt, and ¼ teaspoon black pepper.
* Bring the sauce to a gentle simmer, stirring frequently until the cream cheese is fully melted and incorporated, and the sauce is smooth and slightly thickened (about 5-7 minutes). Taste and adjust seasonings if needed.

4. Combine Pasta and Sauce:
* Add the cooked and drained spaghetti to the skillet with the cheesy tomato sauce.
* Toss gently but thoroughly until all the spaghetti is evenly coated with the sauce.

5. Assemble the Bake:
* Preheat your oven to 375°F (190°C).
* Lightly grease a 9×13 inch baking dish (or a similar-sized casserole dish).
* Transfer the sauced spaghetti mixture to the prepared baking dish, spreading it out into an even layer.

6. Add the Cheesy Topping:
* In a medium bowl, combine the 2 cups of shredded mozzarella cheese and 1 cup of shredded cheddar cheese.
* Sprinkle the mixed mozzarella and cheddar cheeses evenly over the top of the spaghetti in the baking dish.
* Sprinkle the ¼ cup of grated Parmesan cheese over the mozzarella and cheddar.

7. Bake to Golden Perfection:
* Place the baking dish in the preheated oven.
* Bake for 20-25 minutes, or until the cheese is fully melted, bubbly, and lightly golden brown in spots, and the sauce is heated through.
* If you prefer a more browned cheese topping, you can switch the oven to broil for the last 1-2 minutes, but watch it VERY carefully to prevent burning.

8. Rest and Serve:
* Carefully remove the Easy Cheesy Spaghetti from the oven.
* Let it rest for 5-10 minutes before serving. This allows the cheese to set slightly, making it easier to serve and preventing it from being overly runny.