Heat the oil in a pan and cook the onion and chilli until the onion is deep gold and softened – about 15-20 minutes.
Add the garlic and cook for another few minutes then pour in the tomatoes.
Break the tomatoes up with a wooden spoon and stir in the sugar. Pour in the stock and season, then leave to simmer for 15-20 minutes.
Leave to cool a little, then either blitz in a food processor or with a hand-held blender until smooth. Return to the pan and reheat gently.
Make the garnish: Arrange the avocado, feta cheese, coriander, sour cream and lime wedges on small plates.
Heat the oil in a deep sided pan. Cut the tortillas into thin strips and fry until crisp and golden. Drain on kitchen paper.
Serve the soup in bowls with the garnishes laid out for people to help themselves.