Combine onions, chiles, oregano, tomato sauce, and broth in 1- 0.5-quart microproof baking dish. Cover and cook on HIGH (maximum power) 10 minutes; stir after 5 minutes. Add chicken; stir. Set aside. Combine rice and sour cream in shallow greased 2- 0.5-quart microproof baking dish. Sprinkle with 1 cup Cheddar cheese; pour chicken mixture over cheese. Top with remaining 1 cup Cheddar cheese. Cook uncovered on HIGH 5 minutes. Each serving provides: 499 calories 23.9 g. protein 28.8 g. fat 37.3 g. carbohydrate 1012 mg. sodium 82 mg. cholesterol