Preheat your oven to 375°F (190°C). Lightly grease a 9x13 inch baking dish. Cook the spaghetti according to package directions for al dente. Drain well and set aside.

While the pasta cooks, melt 4 tbsp of butter in a large skillet or Dutch oven over medium-high heat. Add the sliced mushrooms and cook, stirring occasionally, until browned and they have released their liquid, about 5-7 minutes. Add the chopped onion and cook until softened, about 3-4 minutes. Stir in the minced garlic and cook for one more minute until fragrant.

Sprinkle the flour over the cooked vegetables in the skillet. Stir constantly for 1 minute to cook out the raw flour taste. This creates a roux that will thicken the sauce.

Gradually whisk in the warm chicken broth and the dry sherry, smoothing out any lumps. Bring the mixture to a simmer and cook for 2-3 minutes, stirring frequently, until the sauce begins to thicken.

Reduce the heat to low and stir in the heavy cream, 1 cup of Parmesan cheese, chopped fresh parsley, salt, pepper, and nutmeg. Continue to stir gently until the cheese is fully melted and the sauce is smooth and creamy. Taste and adjust seasoning if needed.

Add the drained spaghetti and the shredded cooked chicken to the skillet with the cream sauce. Use tongs to gently toss everything together until the pasta and chicken are evenly coated.

Pour the entire mixture into your prepared 9x13 inch baking dish and spread it into an even layer.

In a small bowl, prepare the topping by combining the panko breadcrumbs, 1/4 cup of Parmesan cheese, and 2 tbsp of melted butter. Mix with a fork until the breadcrumbs are evenly moistened.

Sprinkle the breadcrumb topping evenly over the casserole. Bake, uncovered, for 20-25 minutes, or until the sauce is hot and bubbly around the edges and the topping is golden brown and crisp.

Remove the Chicken Tetrazzini from the oven and let it rest for 5-10 minutes before serving. This allows the sauce to set up slightly, making it easier to serve. Garnish with more fresh parsley if desired.