Cook the Pasta:

• Preheat the oven to 350°F (175°C).
• Cook the spaghetti in salted boiling water until al dente. Drain and set aside.

Prepare the Sauce:

• In a large skillet over medium heat, melt the butter.
• Sauté the chopped onion and sliced mushrooms until softened, about 5-7 minutes.
• Add the minced garlic and cook for an additional 1 minute.
• Sprinkle the flour over the vegetables, stirring constantly for 1-2 minutes to eliminate the raw flour taste.
• Gradually whisk in the chicken broth and heavy cream, bringing the mixture to a simmer until it thickens, about 3-5 minutes.
• Stir in the grated Parmesan cheese and dried thyme. Season with salt and freshly ground black pepper to taste.

Combine and Bake:

• Add the cooked spaghetti and shredded chicken to the sauce, tossing to ensure even coating.
• Transfer the mixture to a greased 9×13-inch baking dish.
• Sprinkle the shredded mozzarella cheese evenly over the top.
• Bake in the preheated oven for 25-30 minutes, or until the cheese is melted and bubbly.

Serve:

• Remove from the oven and let it rest for a few minutes.
• Garnish with chopped fresh parsley before serving.