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