Preheat your oven to 350°F (175°C) and lightly grease a 9×13-inch baking dish.
In a large mixing bowl, combine the shredded chicken, cooked spaghetti, chopped celery, onion, and bell pepper. Toss until evenly mixed.
In a separate bowl, whisk together the chicken broth, cream of chicken soup, and salt and pepper until smooth.
Pour the soup mixture over the chicken and spaghetti mixture. Gently stir until everything is well coated.
Transfer the mixture to the prepared baking dish and spread it out evenly.
Sprinkle the shredded cheddar cheese evenly over the top.
Cover the dish with aluminum foil and bake for 35 minutes.
Remove the foil and continue baking for an additional 10 minutes, or until the cheese is melted and bubbly.
Let the casserole rest for a few minutes before serving.
Enjoy!