1. Cook pasta according to package directions, omitting salt and fat. Drain; set aside.
2. Heat a Dutch oven over medium-high heat. Coat pan with cooking spray. Add turkey; cook 3 minutes, stirring to crumble. Add 1 cup onion, bell pepper, and garlic; sauté 3 minutes. Stir in chili powder and next 5 ingredients; cook 1 minute. Add broth, beans, and tomatoes; bring to a boil. Cover, reduce heat, and simmer 20 minutes, stirring occasionally. Remove from heat; stir in chocolate and salt. Serve chili over spaghetti; top with remaining 1/2 cup onion and cheese.