Cook the chicken and onion in a large soup pot. De-bone the chicken (if necessary) and add the next six ingredients to it. After the soup has cooked for a while, add some cut-up tortillas. To serve, top each bowl of soup with grated cheese, tomatoes and green onion.

Variation: Delete the cut-up flour tortillas and serve the soup topped with crushed tortilla chips, cheese and a dollop of sour cream.