In 12-inch skillet, cook beef and onion over medium-high heat 8 to 10 minutes, stirring occasionally, until beef is thoroughly cooked; drain.
Spray 3 1/2- to 4-quart slow cooker with cooking spray. In cooker, mix beef and remaining ingredients except spaghetti.
Cover; cook on Low heat setting 7 to 9 hours.
About 15 minutes before serving, cook spaghetti as directed on box. Spoon chili over spaghetti; serve with shredded Cheddar cheese if desired.