Cut the lettuce into bite-sized pieces and put them into a large salad bowl. Add the beans, avocado, tomatoes and onion with the coriander and chilli (if used). Mix all the ingredients together.
Make the dressing by whisking together the lime juice and oil. Drizzle the dressing over the salad and mix lightly to combine. Garnish with lightly crushed corn chips and serve with soured cream (if liked).