Step 1: Preheat your oven to 375°F (190°C). Lightly grease a 9×13 inch baking dish.
Step 2: Cook the spaghetti or linguine according to package directions, aiming for al dente. Drain well and set aside.
Step 3: In a large pot or Dutch oven, melt the butter over medium heat. Add the chopped onion and sauté until softened, about 5 minutes. Stir in the sliced mushrooms and cook until they release their moisture and start to brown, about 5-7 minutes.
Step 4: Whisk in the minced garlic and cook for 1 minute until fragrant. Stir in the all-purpose flour and cook for another minute, stirring constantly, to create a roux.
Step 5: Gradually whisk in the chicken broth and milk, stirring continuously until the mixture is smooth and begins to thicken. Add the cream of mushroom soup, dry sherry or white wine (if using), and bring to a gentle simmer. Season with salt, black pepper, and poultry seasoning. Taste and adjust seasonings as needed.
Step 6: Remove the pot from heat. Gently fold in the cooked turkey, cooked pasta, and half (1/2 cup) of the grated Parmesan cheese. Stir until all ingredients are well combined and coated with the creamy sauce.
Step 7: Pour the entire mixture into your prepared 9×13 inch baking dish, spreading it out evenly.
Step 8: In a small bowl, combine the remaining 1/2 cup of Parmesan cheese with the panko breadcrumbs. Sprinkle this mixture evenly over the top of the casserole.
Step 9: Bake for 25-30 minutes, or until the casserole is bubbly around the edges and the breadcrumb topping is golden brown and crispy. Let the tetrazzini rest for 5-10 minutes before serving to allow the sauce to set.