Heat 4 tablespoons of the oil in a large saucepan, add the onion and cook for 5 minutes until softened. Add the garlic, cumin and polenta and cook for a further 1 minute.
Stir in the tomatoes, stock, chipotle sauce and sugar and season. Bring to the boil, then reduce the heat and simmer for 15 minutes.
Meanwhile, heat the remaining oil in a frying pan, add the tortilla strips and cook for 1–2 minutes until crisp and golden. Remove with a slotted spoon and drain on kitchen paper.
Whizz the soup until smooth in a blender or using a hand-held stick blender. Stir half the chicken into the soup, then ladle the soup into bowls. Top with the remaining chicken, tortilla strips and avocado. To serve, squeeze over some juice from the wedges and scatter with coriander leaves.