Preheat the oven to 350°F.
Cook the spaghetti in lightly salted water according to package directions until al dente. Drain and set aside.
Mix cooked chicken, cream of mushroom soup, chicken broth, sour cream, Parmesan, diced bell pepper, garlic powder, onion powder, salt, and pepper together in a large bowl. Stir until combined.
Fold the cooked spaghetti into the chicken mixture until evenly distributed.
Pour the mixture into a greased 9x13-inch baking dish, spreading it out evenly.
Sprinkle the shredded cheddar cheese over the top of the casserole.
Bake in the preheated oven for 40-50 minutes, or until the casserole is bubbly and the cheese is golden brown.
Remove from oven and let cool for a few minutes before serving. Sprinkle with fresh parsley for garnish.