01 - Preheat the oven to 375°F. Grease a large 3-quart baking dish with butter or cooking spray.
02 - Cook the pasta in salted boiling water until just al dente, about 1–2 minutes less than package directions. Drain and set aside.
03 - In a large skillet over medium heat, melt 2 tbsp butter. Add mushrooms and cook until browned, about 5 minutes. Add onion and garlic; sauté until soft, 3–4 minutes.
04 - Stir in the remaining butter. Sprinkle flour over vegetables, stirring constantly for 1–2 minutes to cook out the raw flour taste.
05 - Gradually whisk in milk, broth, and cream. Bring to a simmer and cook, stirring constantly, until thickened, 4–5 minutes.
06 - Remove from heat. Stir in Parmesan, salt, pepper, and nutmeg until cheese melts completely.
07 - Add cooked turkey, peas (if using), and pasta to the sauce. Toss to coat evenly.
08 - Transfer mixture to the prepared baking dish, spreading evenly.
09 - Combine breadcrumbs, Parmesan, and melted butter in a bowl. Sprinkle evenly over the casserole.
10 - Bake 25–30 minutes until bubbly and golden on top. Let rest for 10 minutes before serving. Garnish with parsley if desired.