1. Preheat the oven to 350°F (175°C).
2. Cook the spaghetti in salted boiling water until al dente, then drain and set aside, keeping the pot warm.
3. In a skillet, heat olive oil over medium heat and sauté minced garlic with the ground beef until browned and cooked through. Drain any excess fat.
4. Stir in the marinara or spaghetti sauce, Italian seasoning (if using), and diced tomatoes (if desired), then simmer briefly.
5. In the warm pot, add the cubed cream cheese and stir until mostly melted.
6. Add the cooked spaghetti to the cream cheese, mixing until well coated. Then combine the spaghetti mixture with the meat sauce, ensuring even distribution.
7. Transfer the combined mixture to a greased 9×13-inch (23×33 cm) baking dish. Sprinkle half of the shredded cheese evenly on top.
8. Bake uncovered for 20 to 30 minutes until the cheese is bubbly and golden. In the last 5 minutes, sprinkle the remaining cheese and continue baking until melted and golden.
9. Let the casserole rest for about 10 to 15 minutes before serving for better texture and cleaner slices. Garnish with fresh basil if desired.