Preheat your oven to 350°F (175°C).
Cook spaghetti according to package instructions until al dente. Drain and set aside.
In a large skillet, melt the butter over medium heat. Add the sliced mushrooms and cook until they are tender.
Sprinkle the flour over the mushrooms and stir to form a roux; cook for about 2 minutes.
Gradually whisk in the chicken broth and heavy cream, stirring until the mixture is smooth and thickened.
Stir in shredded chicken, garlic powder, salt, pepper, and frozen peas.
Combine the cooked spaghetti with the sauce mixture and stir until well coated.
Transfer the mixture to a greased baking dish. Top with grated Parmesan cheese and breadcrumbs.
Bake for 25-30 minutes or until the top is golden and bubbly.
Allow to cool slightly before serving. Garnish with parsley if desired.