Half fill a medium wok or deep frying pan with water, sprinkle in half of the fish sauce and bring to the boil. Turn down the heat to a simmer, lower the chicken into the water and cover the pan with a lid. Simmer for 10 minutes until the chicken is cooked through. Lift the chicken out of the water and leave until cool enough to handle.
Meanwhile, mix the remaining fish sauce in a large bowl with the lime juice, sugar, pepper and chopped chilli. Tip in the cabbage, carrots and onion. Mix well.
Shred the chicken and toss it into the salad with the herbs. Mix again, cover and leave to marinate in the fridge for at least 2 hours (overnight is ideal).
To make the nuoc cham sauce, pound the garlic, chillies and caster sugar to a paste using a pestle and mortar. Mix in the fish sauce, vinegar and 6 tbsp water. (It will keep for up to 4 days in the fridge.)
To serve, toss the salad, taste and add more sugar, if you like. Scatter mint leaves on each plates, pile the salad on top and sprinkle with peanuts. Serve with individual bowls of nuoc cham sauce to pour over.