Add ground beef and onion to a nonstick skillet over medium-high heat. Break the ground beef apart as it cooks. Cook until there is no more pink in the ground beef.Drain off any grease.
Transfer ground beef mixture to a 4 to 6-quart slow cooker.
Pour enchilada sauce over ground beef. Add velveeta cheese cubes.
Cover and cook on LOW for 1/12 to 2 hours, stirring halfway through.