Cook the spaghetti in salted water until just al dente. Drain well and toss with butter to prevent sticking.

Brown the ground beef and diced onion in a large skillet over medium heat. Drain excess grease if needed. Add garlic and cook for about 30 seconds. Stir in the marinara sauce, Italian seasoning, garlic powder, salt, and pepper. Simmer for 5–10 minutes.

In a bowl, combine softened cream cheese and sour cream until smooth. Stir in about ½ cup of Parmesan.

Grease a 9×13-inch baking dish. Add half of the spaghetti, spread the cream cheese mixture over it, then add half of the meat sauce. Repeat with the remaining spaghetti and sauce. Finish with mozzarella and the remaining Parmesan.

Cover loosely with foil and bake at 350°F (175°C) for 25 minutes. Remove the foil and bake another 10–15 minutes, until the cheese is melted, bubbly, and lightly golden.

Let the casserole rest for 5–10 minutes before slicing. Garnish with fresh parsley if desired.