Chop the onion and then saute it in the oil along with the garlic and chilli flakes.
Add the chopped tomatoes and simmer for 8-10 minutes until thickened.
Use an immersion blender to make everything smooth (or transfer to a processor to do it).
Add the stock, chopped up chicken, salt & pepper and simmer for a further 10 minutes.
Now to prepare the soup toppings. Peel and chop the avocado and slice the lime into wedges. Crush a handful of tortilla chips and place in the empty soup bowl. Spoon the soup on top and then add the avocado, sour cream, grated cheese and more crushed tortilla pieces.