In 10-inch skillet, cook beef and onion over medium heat 8 to 10 minutes, stirring occasionally, until beef is thoroughly cooked; drain. Stir in beans and enchilada sauce. In 3 1/2- to 4 1/2-quart slow cooker, place beef mixture.
In small bowl, mix cornbread mix, milk, melted butter and egg; stir just until moistened. (Batter will be lumpy.) Add cheese and chiles; stir gently to mix. Spoon over beef mixture in slow cooker.
Cover; cook on Low heat setting 5 to 6 hours or until toothpick inserted in center of cornbread comes out clean. Top individual servings with sour cream and onions.