Preheat oven to 350 degrees Fahrenheit.
In a large skillet over medium/high heat, sauté the onions and celery in the olive oil for 4-5 minutes until the onions are translucent.
Add the leftover turkey into the skillet along with the mushrooms (cooked or raw), garlic, poultry seasoning, salt and pepper. Stir well. Cook 2-3 minutes and then remove to a bowl and keep warm.
Melt the butter in the same skillet, and add the flour. Stir well to make a roux and cook for 1-2 minutes.
Whisk in the warmed chicken or turkey stock and continue cooking and stirring until the sauce thickens (about 5 minutes).
Add the heavy cream, cooked pasta, and the turkey mixture back into the skillet.
Stir in ¾ cup Parmesan cheese and most of the chopped parsley (reserving 2 tablespoons for topping). Stir well to combine. Thin the sauce with reserved pasta cooking liquid, if desired.
Spoon the turkey and pasta evenly into a 3 quart casserole dish that has been sprayed with cooking spray. Top with the remaining Parmesan cheese.
Bake uncovered for 30 minutes. Before serving sprinkle with the reserved chopped parsley.