In a large stockpot on medium high heat, sauté onions, red capsicums, celery, lemongrass, chillies and ginger in sesame oil. Add chicken stock, bring to the boil, then lower heat to medium and simmer for 5 minutes.
Add lime leaves, oyster sauce, soy sauce, tom yum paste and chicken. Simmer for 20 minutes and then check the seasoning. Add more oyster sauce, soy sauce or tom yum paste if necessary. Season with salt.