01 - Preheat oven to 375°F. Grease a 9x13 inch baking dish with butter or cooking spray.
02 - Boil pasta in salted water until al dente according to package directions. Drain thoroughly and set aside.
03 - Melt 4 tbsp butter in a large skillet over medium heat. Add onions and mushrooms, sautéing until softened and golden, about 6-8 minutes. Add garlic and cook 1 minute more.
04 - Sprinkle flour evenly over vegetables, stirring constantly for 1 minute to cook out raw flour taste.
05 - Gradually whisk in milk and broth, stirring continuously to prevent lumps. Bring to a gentle simmer and cook until thickened, 3-4 minutes.
06 - Stir in heavy cream, salt, pepper, and nutmeg. Remove from heat.
07 - Fold in cooked turkey, frozen peas, 1/2 cup Parmesan, and mozzarella until fully incorporated.
08 - Add cooked pasta to the skillet, tossing to coat evenly with creamy sauce.
09 - Spread mixture evenly into prepared baking dish.
10 - Combine breadcrumbs, remaining 1/2 cup Parmesan, and melted butter in a small bowl. Sprinkle evenly over casserole.
11 - Bake for 25-30 minutes until bubbling and golden brown on top. Let rest 10 minutes before serving to allow sauce to set.