Directions:

1. Preheat oven to 375°F. Bring a large pot of lightly salted water to a boil and cook spaghetti until not quite al dente, about 2 minutes less than the package instructions. Drain spaghetti and transfer to a 13 x 9-inch baking dish. Toss with 1 tablespoon of the butter.
2. While the spaghetti cooks, start the sauce. Melt 2 tablespoons of the butter in a large, deep skillet over medium heat. Add mushrooms and 1 teaspoon salt and cook until mushrooms are lightly browned, stirring occasionally, about 10 minutes. Add onion and cook until softened, about 5 minutes. Add the flour and stir until absorbed, about 1 minute. Whisk in white wine and bring to a boil. Whisk in gravy if using, and 1 cup of stock (or 2 cups if you are not adding gravy). Reduce heat and simmer sauce until it thickens, about 5 minutes, whisking frequently.
3. Whisk in the cream cheese, pepper, thyme, and remaining salt (taste before adding more salt if you are using gravy in your sauce), then stir in turkey, vegetables, Cheddar, and half of the Parmesan. The mixture should be fairly soupy; add more stock if needed to achieve that consistency. Pour turkey mixture over spaghetti and stir to combine. Melt the last tablespoon of butter and add it to a bowl with the panko and remaining 1/2 cup Parmesan. Stir to moisten the panko and cheese with butter, then sprinkle the mixture on top of the spaghetti. Bake casserole until browned on top, about 25 minutes.