Generously spray 8-inch skillet with cooking spray.
Add onion; cook over medium heat about 3 minutes, stirring occasionally, until crisp-tender.
Spray 3- to 31⁄2-quart slow cooker with cooking spray.
In slow cooker, mix crumbles, onion, beans and enchilada sauce.
In medium bowl, stir muffin mix, milk, butter and egg just until moistened (batter will be lumpy).
Stir in cheese and chiles.
Spoon over mixture in slow cooker.
Cover; cook on Low heat setting 4 hours 30 minutes to 5 hours 30 minutes or until toothpick inserted in center of cornbread comes out clean.
(If slow cooker has black liner, do not cook longer than 5 hours 30 minutes or mixture may burn around edge.)