1. In 2-quart bowl, mix ground beef and onion; cover bowl with waxed paper. Cook at high (100% power) 4 to 5 minutes, stirring and breaking up meat into small pieces every 2 minutes. Tilt bowl; skim and discard excess fat. Stir in tomato sauce, water, and chili powder. Cover and cook, at high 4 minutes or until meat mixture thickens, stirring after 2 minutes. Stir in undrained green chilies and shredded cheese.
2. In shallow 1 1/2-quart casserole, place three-fourths of the coarsely broken tacos. Top with meat mixture; cover with casserole lid or large plate. Cook at medium high (70% power) 12 to 14 minutes until meat mixture is hot. Let stand, still covered, 5 minutes. Sprinkle with remaining tacos.