Put the garlic, onion, chile peppers, tomatoes, and cilantro in a food processor or blender. Process until smooth.
Heat the chicken stock and add pureed mixture along with the cumin and 1 teaspoon of salt. Bring to a boil, reduce heat, and simmer for about 1 hour. Stir cornstarch and water mixture into the soup.
Add the chopped chicken to the soup and heat through.
Meanwhile, fry the tortilla strips in vegetable oil until crisp; drain well.
Garnish with the optional toppings and fried tortilla strips.