Bring a large pot of salted water to a boil. Cook the spaghetti according to package instructions until al dente.

In a mixing bowl, combine the cooked chicken, cream of mushroom soup, sour cream, green bell pepper, onion, garlic powder, salt, and black pepper.

Once the spaghetti is cooked, drain it in a colander.

In the same mixing bowl, add the drained spaghetti to the chicken mixture. Stir gently to coat the spaghetti.

Pour the mixture into a greased 9x13-inch baking dish.

Sprinkle the shredded cheddar cheese on top and add breadcrumbs if using.

Bake in a preheated oven at 350°F (175°C) for 30 minutes.

Let the casserole cool for about 5 minutes before serving.