Heat oil in medium saucepan over medium-high heat and cook onion, stirring frequently, until onion is tender, about 4 minutes. Stir in chili powder and cook 30 seconds. Stir in chicken and cook, stirring frequently, 30 seconds.
Stir in tomatoes and water. Bring to a boil over high heat. Reduce heat to low and simmer, stirring occasionally, 10 minutes. Stir in Knorr® Homestyle Stock Chicken until Stock is melted.
To serve, evenly arrange tortilla chips in 4 soup bowls, then top with soup. Garnish, if desired, with shredded cheddar cheese, sour cream, chopped cilantro and/or lime wedges.