Cook and stir beef and onions in large skillet over medium-high heat, stirring to break up meat, 10 minutes or until meat is no longer pink. Drain; transfer beef mixture into slow cooker.

Add kidney beans, tomato, tomato paste, 1 cup reserved bean canning liquid and chili powder to 4 1/2-quart CROCK-POT slow cooker; mix well. Cover, cook on low 10-12 hours or until tomatoes have softened completely.