Cook spaghetti according to package instructions. Drain and set aside.
In a large skillet, heat olive oil over medium heat. Add onion and bell peppers, and sauté until softened.
Sprinkle in the flour and stir until vegetables are coated. Slowly whisk in the chicken broth and milk, stirring constantly.
Add garlic powder, paprika, salt, and pepper. Simmer for 5 minutes until thickened.
Stir in the cooked chicken and 3/4 cup of shredded cheese until well combined.
Add the cooked spaghetti and mix everything together.
Transfer the mixture to a greased baking dish, top with remaining cheese, and bake at 375°F for 20 minutes.
Sprinkle with parsley before serving if desired.