Preheat oven to 400 degrees F (200 degrees C).

In a 2-quart saucepan, melt 3 tablespoons of butter over medium heat.

Stir in 1/4 cup of all-purpose flour and cook for 3 minutes, creating a roux.

Whisk in 2 1/4 cups of chicken broth, 1/4 cup of dry white wine, 1/4 teaspoon of dried thyme, and 1 pinch of ground nutmeg until smooth.

Heat to boiling, whisking constantly to prevent lumps.

Reduce heat and simmer, whisking frequently, for 5 minutes to thicken the sauce.

Stir in 1/2 cup of heavy cream and set the sauce aside.

In a 10-inch skillet, melt the remaining 1 tablespoon of butter over medium heat.

Add 1 small chopped onion and cook until tender, about 5 minutes.

Add 10 oz of sliced mushrooms and cook, stirring occasionally, for 10 minutes longer, until softened.

In a 2-2 1/2-quart shallow casserole dish, combine the cooked 8 oz of linguine, the mushroom mixture, and 12 oz of cooked turkey.

Stir in the prepared sauce and sprinkle with 3 tablespoons of freshly grated Parmesan cheese.

Bake in the preheated oven until bubbly and golden brown, about 30 minutes.

Let stand for 10 minutes before serving.