Put the chicken into a large deep pan or stockpot with the onion and kaffir lime leaves or bay leaves.
Bruise the lemongrass stalks with the back of a knife and add to the pot.
Turn the heat on under the pan, add water to cover, then bring gently up to the boil.
Cover and simmer very gently for about 45 minutes to one hour, then leave to cool in the liquid.
Once cool, take the chicken out, discard the skin, pull the flesh off the bones and tear into bite-sized pieces. Place in a bowl and set aside. (Strain and save the poaching liquid and use it as stock for another dish.)
To make the salad, put the cucumber into a clean bowl with the teaspoon of salt. Mix well and set aside for ten minutes.
Meanwhile, place the sesame seeds into a small frying pan over a moderate heat (don't add oil) and dry-fry until they turn a shade darker, then take them off the heat and transfer immediately to a cool plate to prevent further cooking.
Once the cucumber has been salting for ten minutes, use your hands to squeeze out any excess liquid from the cucumber. Discard the liquid and transfer the cucumber to a large bowl.
Add the celery, carrots and cabbage to the cucumber along with the chicken and most of the sesame seeds, reserving a few for garnish.
For the dressing, peel and roughly chop the garlic then place in a mortar and pestle.
Deseed and roughly chop the chilli and add to the mortar and pestle.
Add half of the sugar to the mortar and pestle and pound the garlic and chilli to a rough paste. Add the rest of the sugar and pound again.
Juice the limes and add to the paste, then add the fish sauce and a splash of cold water and mix together. Taste and adjust the flavours if necessary.
Toss the salad with enough of the dressing to give a lively flavour, starting with about two-thirds of the dressing and adding more if necessary. Add half the mint and coriander leaves and mix thoroughly.
Line a serving plate with the lettuce leaves. Pile the salad onto the lettuce, then top with the remaining coriander and mint and reserved sesame seeds. Serve swiftly.