In 10-inch nonstick skillet, heat oil over medium heat. Add onion; cook 4 to 6 minutes, stirring occasionally, until onion is tender. Add garlic; cook about 1 minute or until garlic is fragrant. Stir in chicken, taco seasoning mix, water, tomatoes and green chilies until well coated. Heat to simmering; cook uncovered 4 to 5 minutes, stirring frequently, until chicken is heated through.