Preheat Oven:
Preheat your oven to 350°F (175°C). Lightly grease a 9×13-inch (23×33 cm) baking dish.

Cook Spaghetti:
Cook the spaghetti according to package instructions until al dente. Drain and set aside.

Prepare Meat Sauce (if using):
In a skillet over medium heat, cook the ground beef or Italian sausage until browned. Drain excess fat. Stir in the marinara sauce and simmer for 5 minutes.

Prepare Cream Cheese Mixture:
In a large bowl, combine the softened cream cheese, sour cream, garlic powder, Italian seasoning, salt, and black pepper. Mix until smooth.

Combine Spaghetti and Cream Cheese Mixture:
Add the cooked spaghetti to the cream cheese mixture and toss until the pasta is well coated.

Assemble Casserole:
Spread a thin layer of the meat sauce (or plain marinara if not using meat) on the bottom of the prepared baking dish. Layer the cream cheese-coated spaghetti over the sauce. Pour the remaining sauce over the spaghetti. Sprinkle the shredded mozzarella and grated Parmesan cheeses evenly on top.

Bake:
Cover the dish with aluminum foil and bake in the preheated oven for 25 minutes. Remove the foil and bake for an additional 5-10 minutes, or until the cheese is melted and bubbly.

Garnish and Serve:
Remove from the oven and let it rest for 5 minutes. Garnish with chopped fresh parsley before serving.