Preheat oven to 400 degrees F. Butter a 4.8-qt (13x9) casserole dish.
Melt 2 tablespoons of butter in a large pot over medium-high heat, until foaming subsides. Add mushrooms with 1/2 teaspoon salt and 1/4 teaspoon pepper, and cook until liquids are released and mostly evaporated, about 5 minutes.
Add 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 2-3 minutes. Whisk in milk, stock, cream, sherry, nutmeg, 1 teaspoon kosher salt, and 1/2 teaspoon black pepper. Continue cooking, whisking, until the sauce is smooth, glossy, and coats the back of a spoon, about 5 minutes.
Whisk in 3/4 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 cooked linguine and pour into the prepared casserole dish.
Melt the remaining tablespoon of butter, stir in Panko breadcrumbs, the remaining 1/4 cup of parmesan, and 1/4 teaspoon black pepper. Sprinkle breadcrumb mixture on top of the Tetrazzini.
Place dish on a baking sheet and bake for 25 minutes, until bread crumbs are browned 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.
Serve hot. The sauce will thicken further as the tetrazzini stands and cools.
For freezer instructions, see the section in the blog post above titled, "Making Tetrazzini for Freezer Meals".