01 - Preheat the oven to 375°F. Grease a 3-quart baking dish with butter or cooking spray.
02 - Cook pasta in salted boiling 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. Add mushrooms and cook until browned, about 5 minutes. Add onion and garlic; sauté until soft, 3-4 minutes.
04 - Stir in remaining butter. Sprinkle flour over vegetables and stir constantly for 1-2 minutes to cook out raw flour taste.
05 - Gradually whisk in milk, broth, and heavy cream. Bring to a simmer and cook, stirring constantly, until thickened, 4-5 minutes.
06 - Remove from heat. Stir in Parmesan cheese, salt, pepper, and nutmeg until cheese is completely melted and sauce is smooth.
07 - Add cooked turkey, peas, and pasta to the sauce. Toss gently until everything is evenly coated.
08 - Pour the mixture into the prepared baking dish and spread evenly.
09 - Combine breadcrumbs, Parmesan cheese, and melted butter in a bowl. Mix until breadcrumbs are evenly coated.
10 - Sprinkle breadcrumb mixture evenly over the casserole. Bake for 25-30 minutes until bubbly and golden brown on top.
11 - Let the tetrazzini rest for 10 minutes before serving. Garnish with chopped fresh parsley if desired.