In a 3 quart saucepan, cook chicken and onion in oil until chicken juices run clear. Stir in cumin. Add broth, corn, and picante sauce; bring to a boil. Reduce heat; cover and simmer for 15 minutes. Stir in peppers and cilantro.
Combine cornstarch and water until smooth; stir into soup. Bring to a boil; cook and stir for 2 minutes or until slightly thickened. Spoon into bowls; top with cheese if desired.