In 10-inch skillet, cook ground beef and garlic, stirring frequently, until beef is thoroughly cooked; drain. Spoon into 3 1/2- to 4-quart slow cooker. Add cheese and tomatoes; mix well.
Cover; cook on Low heat setting 3 to 4 hours, stirring after 2 hours.
Before serving, stir in onions. Serve with tortilla chips.