Preheat the oven to 350°F (175°C). Grease a large casserole dish with butter or nonstick spray.

Heat a large pot or skillet over medium heat. Add a little oil, then add the chicken breasts. Cook 6–7 minutes per side until golden brown and the internal temperature reaches 165°F (74°C). Remove from heat and shred with two forks. Let rest a couple of minutes.

Meanwhile, bring a large pot of salted water to a rolling boil. Add the spaghetti and cook until al dente, about 8–10 minutes. Drain and set aside.

In a large mixing bowl, combine the shredded chicken, cooked spaghetti, cream of chicken soup, chopped bell peppers, paprika, cayenne pepper, garlic powder, and half of the shredded cheddar. Stir until evenly mixed. Taste and add salt if needed.

Spoon the mixture into the prepared casserole dish and spread it evenly. Sprinkle the remaining cheddar cheese across the top.

Bake for 25–30 minutes, or until the casserole is hot and bubbling and the cheese turns golden brown.

Remove from the oven and let rest for 5 minutes. Garnish with sliced green onions and serve.