Step 1: Preheat your oven to 375°F (190°C). Lightly grease a 9x13 inch baking dish.
Step 2: Cook your pasta according to package directions until 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 and translucent, about 5-7 minutes. Add the minced garlic and cook for another minute until fragrant.
Step 4: If using fresh mushrooms, add them to the pot and cook until they release their liquid and start to brown, about 5-7 minutes. If using canned mushrooms, drain them and add them along with the turkey in the next step.
Step 5: Stir in the cream of mushroom soup, milk, chicken broth, sherry (if using), and a pinch of nutmeg. Bring the mixture to a gentle simmer, stirring occasionally, then reduce heat to low. Season with salt and black pepper to taste.
Step 6: Add the cooked turkey and the drained, cooked pasta to the sauce. Stir gently until everything is well combined and coated in the creamy sauce.
Step 7: Pour half of the pasta mixture into the prepared baking dish. Sprinkle with half of the shredded Parmesan cheese. Add the remaining pasta mixture, spreading it evenly. Top with the remaining Parmesan cheese and the breadcrumbs.
Step 8: Bake for 25-30 minutes, or until the casserole is bubbling around the edges and the topping is golden brown. For an extra crispy top, you can broil for the last 2-3 minutes, watching carefully to prevent burning.
Step 9: Remove from the oven and let rest for 5-10 minutes before serving. Garnish with fresh parsley if desired.