Preheat your oven to 350°F (175°C).
Cook the spaghetti or fettuccine according to package instructions until al dente. Drain and set aside.
In a large skillet, heat the olive oil over medium heat. Add the diced onion and sauté until soft, about 3-4 minutes.
Add the sliced mushrooms to the skillet and cook for another 5 minutes until they are tender.
In a mixing bowl, combine the chicken broth, heavy cream, Parmesan cheese, garlic powder, dried thyme, salt, and pepper. Whisk until well combined.
Add the cooked chicken, drained pasta, sautéed vegetables, and frozen peas to the mixing bowl. Stir until everything is well coated with the sauce.
Transfer the mixture into a greased baking dish. If using, mix breadcrumbs and melted butter in a small bowl and sprinkle over the top of the casserole.
Bake in the preheated oven for 25-30 minutes until the top is golden brown and bubbling.
Remove from the oven and let it cool for a few minutes before serving.