Tom yum gai is a light, fragrant Thai soup. Using aromatics like lemongrass and kaffir lime leaves boosts flavour without adding fat. This method for poaching chicken keeps it nice and moist — as soon as the stock boils, turn the heat off and leave the chicken to cook in the residual heat of the stock, but make sure you don’t leave it for too long as you want to avoid overcooking the meat.
In a large pot, combine chicken stock and raw chicken breasts. Cover and bring to a gentle boil. As soon as it has come to a boil, turn off heat and leave to stand, covered, for 15 minutes or until chicken breasts are just cooked through (they will take 12-18 minutes depending on their size) — you can check they are cooked by cutting in with a sharp knife.
Remove chicken breasts from stock and set aside to rest for a few minutes. If there is any foam floating in the soup, skim it off with a spoon. Add garlic, ginger, lemongrass, kaffir lime leaves, lime juice, fish sauce and sugar. Simmer broth for 5-10minutes.
Shred chicken meat and add back to broth. If needed, season to taste with more fish sauce, sugar and lime juice. Just before serving, stir in coriander, chillies and spring onions. Ladle soup into bowls and serve piping hot.