Remove the skin from the chicken thighs and discard; place the thighs in a large pan. Bruise the lemon grass with a rolling pin and crumple the lime leaves in the palm of your hand. Place in the pan with the garlic, chilli, including the seeds, and ginger and 1.5 litres of water. Bring to the boil and simmer for 30 minutes until the chicken is cooked right through.
Strain the stock into a clean pan then, using 2 forks, shred the meat off the bones. Return the chicken to the pan with the fresh lime juice, soy sauce and sliced salad onions. Heat through gently and when steaming hot, ladle into bowls. Loosely scatter over the coriander leaves and serve.
For pretty presentation and a little extra flavour, try drizzling a splash of chilli or sesame oil over the surface of each serving.