Cook the Pasta:
Preheat your oven to 375°F (190°C). In a large pot of salted boiling water, cook the pasta according to the package instructions until al dente. Drain and set aside.
Sauté the Vegetables:
While the pasta is cooking, heat the olive oil in a large skillet over medium heat. Add the diced onion and sauté for 3-4 minutes until soft. Add the minced garlic and sliced mushrooms, and cook for an additional 3-4 minutes, until the mushrooms are softened and browned.
Make the Sauce:
To the skillet with the onions and mushrooms, add the chicken broth, heavy cream, and whole milk. Stir in the dried thyme, parsley, garlic powder, salt, and pepper. Bring the mixture to a simmer and cook for 5-7 minutes, allowing the sauce to thicken slightly.
Combine Chicken and Pasta:
Stir in the cooked chicken and cooked pasta into the sauce, mixing well to combine. Cook for an additional 2-3 minutes until the chicken is heated through and the pasta is well coated with the sauce.
Assemble the Tetrazzini:
Grease a 9x13-inch baking dish with butter or cooking spray. Transfer the chicken and pasta mixture into the baking dish and spread it out evenly.
Top with Cheese:
Sprinkle the shredded mozzarella cheese and grated Parmesan cheese evenly over the top of the casserole.
Bake:
Place the baking dish in the preheated oven and bake for 20-25 minutes, or until the cheese is melted and bubbly and the top is golden brown.
Serve:
Remove the Chicken Tetrazzini from the oven and let it cool for a few minutes before serving. Garnish with additional parsley or thyme if desired.