Preheat the oven to 350F (175C). Grease a 9x13-inch baking dish.
In a large skillet over medium heat, melt the butter. Add the mushrooms and cook until softened, about 5 minutes.
Stir in the flour and cook for 1 minute, stirring constantly. Gradually add the chicken broth, stirring constantly until the mixture is smooth and thickened.
Add the cooked chicken and cooked spaghetti to the skillet and stir to combine. Add the Parmesan cheese and season with salt and pepper to taste.
Pour the mixture into the prepared baking dish. Bake for 25 minutes, or until the top is golden brown and bubbly.
Interesting Facts
Chicken Tetrazzini was created in the early 1900s by the famous chef, Giuseppe Tetrazzini.
This dish is a popular comfort food in the United States.
The dish is often served with a side of garlic bread.