Preheat oven to 400 degrees F. Butter a 4.8-qt (13x9) casserole dish.

While making the Tetrazzini sauce (instructions follow), cook the pasta in a separate pot of salted boiling water just until al-dente. Drain well.

Melt 2 tablespoons of butter in a large, heavy-bottomed pot over medium-high heat, until foaming subsides. Add mushrooms with ½ teaspoon salt and ¼ teaspoon pepper, and cook until liquids are released and mostly evaporated, about 5 minutes.

Add onions or shallots and cook for about 3-4 more minutes, until mushrooms are beginning to brown. Stir in garlic and thyme and cook for 1 minute, until fragrant. Remove mixture from pan and set aside.

Melt 4 more tablespoons of butter in the empty pan. Stir in flour and cook for 1-2 minutes. Whisk in broth/stock, milk, cream, sherry, nutmeg, 1 teaspoon kosher salt, and ½ teaspoon black pepper. Continue cooking, whisking, until the sauce is smooth, glossy, and coats the back of a spoon, about 5 minutes.

Whisk in ¾ cup Parmigiano Reggiano until smooth. Remove from heat and fold in sautéed mushrooms, peas, shredded turkey, and parsley. Season to taste with salt and pepper.*

Stir in the cooked pasta and pour the mixture into the prepared casserole dish.

In a bowl, stir together the melted butter, Panko breadcrumbs, parmesan, and ¼ teaspoon black pepper. Sprinkle breadcrumb mixture evenly across the Tetrazzini.

Place casserole dish on a baking sheet and bake for 25 minutes, until the topping is toasted and sauce is bubbling. Watch the breadcrumbs, particularly around the 15-minute mark. If they're starting to brown too quickly, shield the top of the dish with a piece of foil.

Let the Tetrazzini stand for a few minutes after baking. Garnish with parsley and serve hot. The sauce will thicken significantly as it cools.