Preheat your oven to 350°F (175°C) and grease a large casserole dish.
In a large pot over medium heat, cook the chicken for about 6-7 minutes per side until golden brown and cooked through.
Bring salted water to a boil and cook spaghetti until al dente (about 8–10 minutes). Drain the pasta and set aside.
In a mixing bowl, combine shredded chicken, cooked spaghetti, cream of chicken soup, chopped bell peppers, paprika, cayenne pepper, garlic powder, and half of the cheddar cheese.
Transfer the mixture into your greased casserole dish and top with the remaining cheddar cheese.
Place your casserole in the preheated oven and bake for about 25–30 minutes until it's bubbly and golden brown on top.
Let your casserole rest for about 5 minutes before garnishing with sliced green onions.