Spray a large nonstick skillet with cooking spray. Add ground beef and chopped onion and cook and stir over medium hight heat until meat is no longer pink. Drain on a paper-towel lined plate. Pour off any grease from skillet and wipe clean. Return the beef mixture to the skillet and add the torn tortillas and black beans, stirring to mix well. Cover skillet and cook over medium heat for about five minutes, stirring often.