Preheat your oven to 350°F. Generously grease a 9x13-inch casserole dish with butter or cooking spray. Pat chicken breasts dry with paper towels and season both sides with salt and black pepper.

Heat olive oil in a large skillet over medium heat. Cook chicken breasts for 6 to 7 minutes per side until internal temperature reaches 165°F and juices run clear. Remove from heat, let rest for 5 minutes, then shred using two forks.

While chicken cooks, bring a large pot of salted water to a boil. Cook spaghetti for 8 to 10 minutes until al dente. Drain well and set aside.

In a large mixing bowl, combine shredded chicken, cooked spaghetti, cream of chicken soup, chopped bell peppers, paprika, cayenne pepper, garlic powder, and 1 cup of the shredded cheddar cheese. Mix thoroughly until everything is evenly coated.

Transfer mixture to prepared casserole dish, spreading evenly. Top with remaining 1 cup of cheese, covering the entire surface.

Bake uncovered for 25 to 30 minutes until cheese is melted, bubbly, and shows light golden spots around edges. Internal temperature should reach 165°F. Let rest for 5 minutes before garnishing with sliced green onions and serving.