Preheat oven to 350. Prepare a 13" x 9" pan with cooking spray. Place cooked spaghetti in prepared pan. Meanwhile, in a large saucepan, cook chicken, onions, and black pepper until chicken is no longer pink. Stir in chili with beans, tomatoes with juice, one cup cheese, sour cream, chili powder, and salt. Mix well. Add chili mixture to spaghetti in pan; stirring until well coated. Sprinkle with remaining cheese. Cover tightly with foil and bake 30 minutes, or until hot and bubbly. Let stand five minutes before serving.