01 - Preheat oven to 375°F. Grease a 9x13 inch baking dish with butter or cooking spray.
02 - Cook pasta in salted boiling water until al dente according to package directions. Drain well and set aside.
03 - Melt 4 tbsp butter in a large skillet over medium heat. Add onions and mushrooms, sauté until softened and golden, about 6-8 minutes. Add garlic and cook 1 minute more.
04 - Sprinkle flour over the vegetables, stirring constantly for 1 minute to cook out the raw flour taste.
05 - Gradually whisk in milk and broth, stirring constantly to prevent lumps. Bring to a simmer and cook until thickened, 3-4 minutes.
06 - Stir in heavy cream, salt, black pepper, and nutmeg. Remove from heat.
07 - Fold in turkey, peas, 1/2 cup Parmesan cheese, and mozzarella cheese until well combined.
08 - Add cooked pasta to the sauce mixture and toss until evenly coated.
09 - Pour the mixture into the prepared baking dish, spreading evenly.
10 - In a small bowl, combine breadcrumbs, remaining 1/2 cup Parmesan cheese, and melted butter. Mix until crumbs are evenly coated.
11 - Sprinkle topping evenly over the casserole. Bake for 25-30 minutes until bubbly and golden brown on top.
12 - Let the casserole rest for 10 minutes before serving to allow the sauce to set slightly.