Heat the oil in a large saucepan, add the onion and cook for 5 minutes until softened. Add the chilli and garlic, then pour in the chicken stock and bring to the boil. Reduce the heat and simmer for 10 minutes. Whizz the stock until smooth using a hand-held stick blender, then leave to cool completely.
When ready to serve, stone and peel the avocados, then place in a food processor or blender with the cooled stock and whizz until smooth. Stir in the lime juice and season to taste.
Ladle the soup into bowls, then top with the prawns. Scatter with the coriander and sprinkle over some crushed ice to serve.