Preheat the oven to 350 degrees F. Coat a 9"x13" pan with cooking spray.
Cook the spaghetti in a large pot of salted water according to package directions.
Melt 4 tablespoons of the butter in a large skillet over medium heat. Add the onion and cook for 3-4 minutes or until softened. Add the mushrooms and cook for another 4-5 minutes until the vegetables are browned and tender.
Add the garlic and cook for 30 seconds. Stir in the flour and cook for 1 minute.
Slowly add the chicken broth and half and half to the pan, whisking constantly.
Bring the milk mixture to a simmer and cook for 3-4 minutes or until just thickened.
Remove the sauce from the heat, and mix in the parmesan cheese, cream cheese and Italian seasoning. Stir until well blended. Season with salt and pepper to taste.
Drain the spaghetti and return it to the cooking pot or place it in a large bowl. Add the chicken and peas to the pasta.
Pour the sauce over the pasta and toss to coat evenly.
Place the pasta mixture into the prepared pan.
Melt the remaining 2 tablespoons of butter. Pour the butter into the breadcrumbs and mix to combine. Sprinkle the breadcrumbs over the pasta.
Bake for 20-25 minutes, until casserole is bubbly and breadcrumbs are browned. Sprinkle with parsley, then serve.