1. Place onion and garlic in 2-quart casserole. Cover tightly and microwave on high 3 to 4 minutes or until onion is tender. Stir in broth, chili powder and tomato sauce. Cover tightly and microwave 8 to 10 minutes, stirring after 4 minutes, until boiling.
2. Divide chicken and tortilla chips among 4 soup bowls. Pour soup over chips; sprinkle with cheese. Microwave uncovered on high 1 to 2 minutes or until cheese begins to melt. Top with avocado.