Preheat oven to 350°. In a large skillet, cook beef and onion over medium heat 6-8 minutes or until beef is no longer pink and onion is tender, breaking up beef into crumbles; drain. Stir in beans; heat through., Reserve 1 cup corn chips for topping. Place remaining corn chips in a greased 13x9-in. baking dish. Layer with meat mixture, enchilada sauce and cheese; top with reserved chips., Bake, uncovered, 15-20 minutes or until cheese is melted. If desired, sprinkle with green onions.