Make the guacamole. Roughly chop the avocado flesh and mash it together with the spring onions, tomatoes, coriander and lime juice. Set aside.
Heat the oil in a medium saucepan. Add the onion, garlic and chilli and fry for 2–3 minutes or until softened. Add the beans, tomatoes and stock, bring to the boil and simmer for 10 minutes.
Transfer three-quarters of the soup to a food processor or blender and process until almost smooth. Add to the reserved soup and stir to combine. Season to taste with salt and pepper and warm through.
Serve the soup with the guacamole and tortilla crisps.