Place chicken, onion, and lime or bay leaves into a large pot.
Bruise lemongrass stalks and add them to the pot.
Cover the chicken with water and bring it to a gentle boil.
Simmer covered for 45 to 60 minutes and let cool in the liquid.
Shred the cooled chicken meat into bite-sized pieces, discarding skin and bones.
Mix cucumber strips with salt and let sit for 10 minutes.
Dry-fry sesame seeds in a pan until golden and transfer to a plate.
Squeeze excess liquid from the cucumber and place in a large bowl.
Mix chicken, celery, carrots, cabbage, and most sesame seeds into the bowl.
Roughly chop the garlic for the dressing.
Deseed and chop the chilli and add to the mortar.
Pound garlic, chilli, and sugar into a rough paste.
Mix in lime juice, fish sauce, and a splash of water.
Toss the salad with dressing and half of the fresh herbs.
Serve salad over lettuce leaves topped with remaining herbs and seeds.