Prepare the Ingredients:

1. Cook the Pasta: Begin by cooking the pasta according to the package instructions in a large pot of salted water. Be sure to cook the pasta al dente, as it will continue to cook in the oven later. Once cooked, drain the pasta and set it aside.
2. Cook the Chicken: If you haven’t already cooked the chicken, you can quickly do so by pan-searing chicken breasts in a bit of olive oil over medium heat for about 6-8 minutes on each side, or until the internal temperature reaches 165°F (75°C). Let the chicken rest for a few minutes, then shred it into bite-sized pieces. Alternatively, you can use rotisserie chicken for a quicker option.
3. Sauté the Vegetables: In a large skillet, melt the butter over medium heat. Add the chopped onions and cook for 2-3 minutes until they become translucent. Add the garlic and cook for another 30 seconds until fragrant. Stir in the sliced mushrooms and cook for 5-7 minutes until they release their moisture and become tender.

Make the Sauce:

4. Prepare the Roux: In the same skillet with the mushrooms and onions, sprinkle the flour over the vegetables and stir to combine. Cook the flour for 1-2 minutes, stirring constantly, to create a roux. This step thickens the sauce and helps achieve that creamy texture.
5. Add the Liquids: Slowly pour in the chicken broth while stirring to prevent any lumps from forming. Once the broth is incorporated, pour in the heavy cream and stir well. Bring the mixture to a simmer, allowing the sauce to thicken, which should take about 5 minutes.
6. Add the Cheese and Seasoning: Once the sauce has thickened, reduce the heat to low. Stir in 1 cup of grated Parmesan cheese and 1 cup of shredded mozzarella cheese until the cheese is melted and the sauce is smooth. Season with salt, pepper, thyme, basil, and nutmeg (if using), and taste for seasoning. Adjust with more salt and pepper if necessary.

Combine and Bake:

7. Assemble the Tetrazzini: Add the cooked pasta and shredded chicken to the skillet with the sauce, tossing to combine until everything is coated in the creamy mixture.
8. Transfer to Baking Dish: Pour the pasta and chicken mixture into a greased 9×13-inch baking dish. Spread it out evenly and top with the remaining ½ cup of grated Parmesan cheese.
9. Bake: Preheat your oven to 375°F (190°C) and bake the Chicken Tetrazzini for 20-25 minutes or until the top is golden brown and the casserole is bubbling around the edges. If you like a crispier top, you can broil the casserole for the last 2-3 minutes of baking, but be sure to keep an eye on it to prevent burning.

Garnish and Serve:

10. Garnish and Serve: Once the Chicken Tetrazzini is out of the oven, sprinkle with freshly chopped parsley for a touch of color and flavor. Let the dish rest for a few minutes before serving.