Preheat the oven to 350 degrees F. Coat a 9"x13" baking dish with cooking spray.
Boil the spaghetti in a pot of salted water according to the package directions.
Melt 4 tablespoons of the butter in a large skillet over medium heat. Add the onion and saute for 3-4 minutes.
Add the mushrooms and cook for another 4-5 minutes until all of 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 then the half and half to the pan, whisking the whole time.
Bring the sauce mixture to a simmer and cook for 3-4 minutes or until it has just thickened.
Take the sauce off the heat, and mix in the parmesan cheese, cream cheese and Italian seasoning. Stir until well blended and cheese is melted. Season with salt and pepper to taste.
Drain the spaghetti and place it in a large bowl. Add the turkey and peas to the pasta.
Pour the mushroom 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. Sprinkle the breadcrumbs over the top of the pasta.
Bake for 20-25 minutes, until the breadcrumbs are browned. Sprinkle with parsley, then serve.