The broth is made from tomatoes, shallots, garlic and a little fermented soybean paste, simmered together until it smells wonderful. It is light orange in colour and has a gentle sourness from the tomatoes and a depth of flavour from the slow cooking. Fresh rice noodles are added just before serving.
On top goes a generous pile of fresh herbs - coriander, spring onions and mint - plus a squeeze of lime and sometimes a fried shallot crisp for crunch. The contrast between the warm broth, slippery noodles and cold fresh herbs is part of what makes the dish interesting.
Khao soi is most popular in the morning and at lunchtime. Noodle soup stalls open very early and are often surrounded by motorbikes and tuk-tuks while people eat a quick, warming bowl before work or school.
It is a good example of how the same name ('khao soi') can mean very different dishes in different countries. The Thai version and the Lao version were probably both inspired by the same idea long ago, but evolved differently over generations depending on which local ingredients were available.