
Preheat the oven to 375 degrees F.
Bring a large pot of salted water to a boil. Add the spaghetti and cook until al dente, 7 to 8 minutes. Drain and set aside.
Meanwhile, heat the butter in a large ovenproof skillet over medium heat. When the butter has melted, add the celery, carrot, sage, garlic and onion, then stir. Season with the pepper and a pinch of salt and cook for 2 to 3 minutes to soften the vegetables. Add the turkey, cream of mushroom soup, gravy and stock. Bring to a simmer.
(OPTIONAL) WE DID NOT BAKE IT. Stir in the pasta and half of the Cheddar until well combined. Sprinkle the remaining Cheddar over the top and bake until the cheese is melted and the sides are beginning to brown, 12 to 15 minutes. Let cool slightly, then garnish with the parsley and serve.