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

In a large pot, cover chicken breasts with water, bring to a simmer over medium heat, and cook 6-7 minutes per side until internal temperature reaches 165°F (74°C). Remove chicken and shred.

In another pot, boil salted water and cook spaghetti until al dente, approx. 8-10 minutes. Drain and set aside in a large mixing bowl.

Add shredded chicken, cream of chicken soup, chopped bell peppers, paprika, cayenne pepper, garlic powder, and half of sharp cheddar cheese to the spaghetti and mix well.

Transfer the mixture to the greased casserole dish, top with remaining cheddar cheese, and bake for 25-30 minutes until bubbly and golden.

Allow the casserole to rest for 5 minutes before garnishing with chopped green onions and serving.