First Step: Prepare all ingredients by cooking and shredding chicken, boiling pasta al dente, and slicing mushrooms. Preheat the oven to 350°F (175°C).

Second Step: In a large skillet, melt butter over medium heat, then add mushrooms and sauté until tender, about 5 minutes.

Third Step: Stir in flour and cook for 1-2 minutes while stirring to create a roux.

Fourth Step: Gradually whisk in chicken broth and milk, cooking until the sauce thickens, about 4-6 minutes.

Fifth Step: Add grated Parmesan cheese to the sauce and season with salt and pepper, then stir in the cooked chicken.

Sixth Step: Combine the sauce with cooked spaghetti and mix thoroughly to coat evenly.

Seventh Step: Transfer the mixture to a greased baking dish and sprinkle additional cheese on top if desired.

Eighth Step: Bake uncovered in the preheated oven for 25-30 minutes or until the top is golden and bubbly.

Final Step: Remove from oven, let cool slightly, then serve warm. Adjust seasoning or garnish with fresh herbs. For dietary adaptations, substitute ingredients accordingly before baking.