Combine onions, chiles, oregano, tomato sauce, and broth in 1-1/2-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 2-1/2-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