Preheat oven to 375℉ and butter a 9x13 inch baking dish or 4-quart casserole dish.
While you are preparing the sauce, cook the noodles in heavily salted boiling water for 1 minute less than the package directions state. Drain noodles, reserving ¼ cup of cooking liquid.
Melt 4 tablespoons of butter in a large saucepan over medium-high heat. Add in sliced mushrooms and cook until brown and most of the liquid evaporates. Add in thyme leaves and garlic, if using, and cook for 1 minute longer.
Sprinkle the mushrooms with flour and cook for 1 minute to get rid of any raw flour taste.
Add in the sherry wine (or equal amounts for dry white wine or chicken stock) and cook until the liquid evaporates.
Slowly add in chicken stock, whisking into the mushrooms. Add in milk, nutmeg, salt, and pepper, and bring the sauce to a simmer, and simmer for 5 minutes. Whisk in Parmesan cheese and turn off the heat.
Add turkey and cooked noodles to the cream sauce and stir to combine. If the mixture seems too thick, add in the reserved pasta sauce.
Pour the turkey and pasta mixture into the prepared casserole dish.
Prepare bread crumb topping by mixing together the bread crumbs, parmesan, thyme, and butter together. Sprinkle that mixture evenly over the turkey casserole.
Bake the turkey tetrazzini until bubbly and golden, for 20-25 minutes.
Video
Notes
Turkey: In place of leftover turkey, use leftover chicken. 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.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. Hate Mushrooms? Omit them! The sauce will be plenty delicious without the mushrooms.Add Another Vegetable. Feel free to add up to 1 cup of frozen mixed vegetables or peas when combining the noodles and turkey into the sauce prior to baking the casserole.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.