Preheat the oven to 400°F (200°C). Lightly grease a 13x9-inch baking dish with cooking spray or butter.
Cook the spaghetti in a large pot of salted boiling water until al dente. Drain well and set aside.
8 ounces dried spaghetti
In a large saucepan over medium heat, melt 6 tablespoons butter. Add the sliced mushrooms and sauté for 4 to 5 minutes, until soft and their liquid has evaporated. Stir in the flour and cook for 1 minute, stirring constantly to form a smooth paste (roux).
8 ounces mushrooms, 8 tablespoons butter, ½ cup all-purpose flour
Gradually whisk in the chicken broth and half-and-half, stirring to prevent lumps. Reduce heat to low and simmer for 4 to 6 minutes, stirring occasionally, until the sauce slightly thickens. Add the chopped turkey and stir until evenly coated in the sauce.
1 ½ cups half-and-half, 2 cups chicken broth, 4 cups chopped cooked turkey
Stir the cooked spaghetti into the sauce, mixing until everything is well combined. Transfer the mixture to the prepared baking dish and spread it evenly.
In a small bowl, combine the bread crumbs and grated Parmesan cheese. Sprinkle this mixture evenly over the casserole. Dot the top with the remaining 2 tablespoons butter.
½ cup grated Parmesan cheese, ½ cup bread crumbs
Bake for 25 to 30 minutes, until the sauce is bubbly and the topping is golden brown and crisp.
Let cool for a few minutes before serving.