01 - Preheat the oven to 375°F. Grease a 3-quart baking dish with butter or cooking spray.
02 - Boil pasta in salted water until al dente, about 1-2 minutes less than package directions. Drain well and set aside.
03 - Melt 2 tbsp butter in a large skillet over medium heat. Cook mushrooms until browned, 5 minutes. Add onion and garlic; sauté until soft, 3-4 minutes.
04 - Stir in remaining butter. Sprinkle flour over vegetables, stirring constantly for 1-2 minutes to cook out 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 is fully melted and smooth.
07 - Add cooked turkey, peas if using, and pasta to the sauce. Toss thoroughly until everything is evenly coated.
08 - Pour mixture into the prepared baking dish, spreading evenly.
09 - Combine breadcrumbs, Parmesan, and melted butter in a bowl. Mix until butter is evenly distributed.
10 - Sprinkle breadcrumb mixture evenly over the casserole.
11 - Bake for 25-30 minutes until bubbly and golden brown on top. Let rest for 10 minutes before serving. Garnish with parsley if desired.