Arrange tortilla strips on a baking tray and bake until lightly browned, 3 to 5 minutes.
In a large saucepan over medium heat, cook onion, garlic and chillies in olive oil until lightly browned, 4 to 5 minutes. Stir in chicken, chicken stock, lime juice, tomato, salt and pepper. Gently simmer until chicken is cooked, 3 to 5 minutes.
Stir in avocado and coriander and heat through. Adjust seasonings.
Ladle soup into bowls and sprinkle with tortilla strips to serve.