Cook the Pasta: Cook the pasta according to the package directions until al dente. Drain and set aside.
- Prepare the Sauce: In a large skillet, heat the olive oil over medium heat. Add the chopped onion and garlic and sauté for about 3-4 minutes until softened and fragrant.
- Add Wine and Broth: If using, pour in the white wine and allow it to simmer for 2-3 minutes until reduced slightly. Add the chicken broth and bring to a simmer.
- Make it Creamy: Reduce the heat to low and stir in the heavy cream, Parmesan cheese, and shredded mozzarella. Mix until the cheese has melted and the sauce is smooth.
- Season: Add the thyme, nutmeg, salt, and pepper, stirring to combine. Taste the sauce and adjust the seasoning as needed.
- Combine Chicken and Pasta: Add the shredded chicken and cooked pasta to the sauce, stirring gently to coat everything in the creamy sauce.
- Assemble the Tetrazzini: Transfer the chicken and pasta mixture into a greased 9×13-inch baking dish.
- Prepare the Topping: In a small bowl, melt the butter and mix it with the breadcrumbs. Sprinkle the breadcrumb mixture evenly over the casserole.
- Bake: Preheat the oven to 375°F (190°C). Bake the Tetrazzini for 20-25 minutes or until the top is golden brown and the sauce is bubbling.
- Serve: Garnish with fresh parsley and serve hot.