In a large stock pot saute onions, red bell peppers, celery, lemongrass, chile peppers and ginger in sesame oil.
Add chicken stock and simmer for 5 minutes.
Add lime leaves, oyster sauce, soy sauce, tom yum paste and cut up chicken. Simmer for 20 minutes and then check the seasoning. Add more oyster sauce, soy sauce or tom yum paste if necessary. At this point also season with salt.