1. Preheat the oven to 375°F and lightly grease a 9×13-inch baking dish.

2. Bring a large pot of salted water to a boil and cook the spaghetti until just al dente. Drain and set aside.

3. In a large skillet, melt the butter over medium heat. Add the onion and mushrooms and cook until softened, about 5 to 6 minutes.

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

5. Sprinkle in the flour and stir well for 1 minute to form a light roux.

6. Slowly whisk in the chicken broth until smooth, then pour in the heavy cream.

7. Add the cream cheese and stir until melted and fully incorporated.

8. Mix in the Parmesan cheese, salt, black pepper, and Italian seasoning. Simmer for 3 to 4 minutes until the sauce thickens slightly.

9. Add the cooked chicken and drained spaghetti to the skillet and toss until evenly coated in the sauce.

10. Transfer the mixture to the prepared baking dish and spread it evenly.

11. Sprinkle the mozzarella cheese over the top.

12. Bake for 20 to 25 minutes until hot and bubbly and the cheese is melted and lightly golden.

13. Let the casserole rest for 5 to 10 minutes, then sprinkle with parsley and serve.