2 oz. (4 Tbs.) unsalted butter; more for the baking dish
3/4 cup coarse dry breadcrumbs
2 Tbs. extra-virgin olive oil
1 Tbs. freshly grated Parmigiano-Reggiano
1 Tbs. finely chopped fresh sage
Freshly ground black pepper
1 lb. assorted fresh mushrooms, cleaned and sliced 1/8 to 1/4 inch thick
3 Tbs. minced shallots
2 medium cloves garlic, minced
2-1/4 oz. (1/2 cup) all-purpose flour
6 cups low-fat milk
3-1/2 oz. (1-1/2 cups) grated sharp white Cheddar
3 Tbs. finely chopped fresh flat-leaf parsley
8 oz. egg pappardelle pasta
3 cups shredded or diced roast turkey
Position a rack in the center of the oven and heat the oven to 350°F.
Butter a 3-quart baking dish. Bring a large pot of well-salted water to a boil over high heat. Mix the breadcrumbs, 1 Tbs. of the olive oil, Parmigiano, and sage in a small bowl. Season to taste with salt and pepper. Set aside.
Heat a large (12-inch) skillet over medium heat. Add the remaining 1 Tbs. olive oil to the pan and then add the mushrooms; cook, stirring frequently, until softened and golden on the edges, about 10 minutes. Reduce the heat to medium low and add the shallots and garlic; cook, stirring until softened, about 2 minutes. Season to taste with salt and pepper and set aside.
Melt the butter in a 4-quart saucepan over medium heat. Add the flour and whisk constantly until it colors slightly, 2 to 3 minutes. Remove the pan from the heat and gradually whisk in enough of the milk to form a thick, smooth paste. Set the pan back over the heat and whisk in the remaining milk in a steady stream. Add 1 tsp. of salt. Bring to a boil over medium-high heat, whisking constantly. Reduce the heat to medium low and simmer for 3 minutes, whisking constantly. Turn off the heat and stir in the Cheddar and parsley. Season to taste with more salt and pepper. Set aside.
Cook the pasta in the boiling water until al dente, about 1 minute less than package timing. Drain and spread the pasta in an even layer on the bottom of the prepared baking dish. Sprinkle the turkey and mushrooms over the pasta. Pour the sauce on top and use a fork to distribute it evenly.
Sprinkle the breadcrumbs over the entire casserole. Set on a baking sheet to catch drips and bake until golden-brown and bubbling, 50 to 60 minutes. Let rest 20 to 30 minutes before serving.