Preheat oven to 375℉/190℃ and butter a 9x13 inch baking dish or 4-quart casserole dish.
While you are preparing the sauce, cook 8 ounces spaghetti noodles in heavily salted boiling water for 1 minute less than the package directions state. Drain noodles, reserving ¼ cup of cooking liquid.
Mel t4 tablespoons unsalted butterin a large saucepan over medium-high heat. Add in 8 ounces button mushrooms (sliced) and cook until brown and most of the liquid evaporates. Add in 2 teaspoons fresh thyme leaves and 2 teaspoons minced garlic, and cook for 1 minute longer.
Sprinkle the mushrooms with ⅓ cup all-purpose flour and cook for 1 minute to get rid of any raw flour taste.
Add in ⅓ cup dry sherry (or equal amounts for dry white wine or chicken stock) and cook until the liquid evaporates.
Slowly whisk in 2 cups low-sodium chicken stock until incorporated.. Add in 2 cups milk, ¼ teaspoon salt, ½ teaspoon black pepper, and ¼ teaspoon grated nutmeg, and bring the sauce to a simmer. Simmer for 5 minutes. Whisk in ½ cup grated parmesan cheese and turn off the heat.
Add 2 cups cooked turkey and the cooked noodles to the cream sauce and stir to combine. If the mixture seems too thick, add in the reserved pasta cooking liquid.
Pour the turkey and pasta mixture into the prepared casserole dish.
Prepare bread crumb topping by mixing together the 1 cup panko bread crumbs, ¼ cup grated parmesan cheese, 3 tablespoons unsalted butter (melted) and ½ teaspoon thyme leaves together. Sprinkle that mixture evenly over the turkey casserole.
Bake the turkey tetrazzini until bubbly and golden, for 20-25 minutes, or until warmed through and golden.
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Notes
Noodles: Use regular or whole wheat spaghetti, rotini, or elbow noodles. Sherry: Use dry sherry for best results. If you don't have sherry use dry white wine, dry marsala, or additional chicken stock in place of the sherry.Bread Crumbs: I recommend panko bread crumbs for the crispiest of toppings, but traditional bread crumbs can be used.Thyme: Do not use ground thyme, as it is way too overpowering. But dried thyme leaves will work. Turkey: Keep in mind that leftover turkey is only safe to consume for up to 4 days after baking unless frozen. It is best to prepare and consume Turkey Tetrazzini within 4 days of preparing turkey OR use defrosted frozen, cooked turkey. Feel free to use ham or chicken in place of the turkey. Leftovers: Leftover turkey tetrazzini should be stored in an airtight container in the refrigerator. It is best to consume Turkey Tetrazzini within 4 days of preparing turkey OR use defrosted frozen, cooked turkey.