Preheat the oven to 350 degrees F (175 degrees C). Lightly grease a 9x13 inch baking dish.

In a large skillet over medium heat, melt the butter and sauté the onions and mushrooms until soft, about 5 minutes. Add garlic and cook for 30 seconds more until fragrant.

In a large mixing bowl, combine the cream of mushroom soup, sour cream, chicken broth, thyme, basil, salt and pepper. Mix well until smooth.

Add the cooked chicken, sautéed vegetables, and 1 cup of mozzarella cheese to the soup mixture. Stir to combine.

Layer half of the cooked spaghetti in the bottom of the prepared baking dish. Pour half of the chicken mixture over the pasta. Repeat with remaining pasta and chicken mixture.

Top with the remaining mozzarella cheese, Parmesan cheese, and breadcrumbs if using.

Bake uncovered for 25-30 minutes until the casserole is bubbling and the top is golden brown.

Let stand for 5-10 minutes before serving to allow the sauce to thicken slightly.