Preheat your oven to 350°F (175°C).
In a large mixing bowl, combine the cooked spaghetti and shredded chicken.
Add the cream of mushroom soup and sour cream to the mixture.
Stir in the shredded cheddar cheese, chopped bell peppers, and onions.
Season the mixture with garlic powder, paprika, salt, and pepper.
Transfer the mixture to a greased casserole dish.
Spread it evenly across the dish.
Top with additional shredded cheese if desired.
Bake in the preheated oven for 30 minutes until bubbly and golden.
Remove from the oven and let it cool for a few minutes before serving.