Preheat your oven to 350°F (175°C) and lightly grease a large casserole dish with cooking spray or olive oil.

In a large pot over medium heat, cook the boneless chicken breasts for about 6-7 minutes on each side, until golden brown.

Bring a large pot of salted water to a boil. Once boiling, cook the spaghetti until al dente, approximately 8-10 minutes.

In a bowl with the cooked spaghetti, add the shredded chicken, cream of chicken soup, chopped bell peppers, paprika, cayenne pepper, and half of the cheddar cheese.

Transfer the mixed ingredients into the greased casserole dish, spreading them out evenly. Top with remaining cheddar cheese.

Place the casserole in the oven and bake for 25-30 minutes, until bubbly and golden brown.

Remove the casserole from the oven and let it rest for about 5 minutes. Garnish with chopped green onions before serving.