Bring the chicken stock, chicken breasts, red onion, garlic, oregano, salt, pepper and thyme to a boil in a large pot; reduce heat to medium-low and simmer until the chicken breasts are no longer pink in the center and the juices run clear, 15 to 20 minutes. Remove the cooked chicken to a cutting board and shred into bite-sized strips; return to the simmering pot.
Heat the oil in a frypan over medium heat; cook the spring onions and chilli in the hot oil until tender, about 5 minutes. Stir the tomatoes into the mixture and continue cooking until soft, about 5 minute more; pour the mixture into the pot with the chicken soup. Season with the salt; return the soup to a simmer. Add the lime juice and 1/2 a lime; cook another 10 minutes. Remove the pot from the heat and remove the lime half; stir in the coriander to serve.