Preheat oven to 350 degrees F. Grease a large casserole dish with cooking spray or butter and set aside.

Place chicken breasts in a large pot of salted water. Bring to a boil over medium-high heat, then reduce to a simmer and cook for 20 to 25 minutes until fully cooked through and internal temperature reaches 165 degrees F. Transfer to a cutting board, rest 5 minutes, then shred with two forks. Reserve a small amount of poaching liquid.

Bring a separate pot of salted water to a boil. Cook spaghetti until just al dente, about 7 to 8 minutes. Drain and do not rinse.

In a large mixing bowl, combine shredded chicken, cooked spaghetti, cream of chicken soup, chopped bell peppers, paprika, cayenne, garlic powder, and 1 cup of the shredded cheddar. Stir until well combined. If the mixture feels stiff, add 2 to 3 tablespoons of reserved poaching liquid to loosen.

Transfer mixture to the prepared casserole dish, spread evenly, and top with the remaining 1 cup of cheddar cheese.

Bake uncovered at 350 degrees F for 25 to 30 minutes until the cheese is fully melted and golden brown around the edges and the casserole is bubbling at the sides. Rest for 5 minutes before garnishing with sliced green onions and serving.