Pour the stock into a large saucepan, add the chilli sauce, fish sauce, soy sauce, rice vinegar and mirin and bring to the boil. Reduce to a simmer, then add the chicken breasts and cook for 10–15 minutes until just cooked through. Remove from the pan and cut into thick slices.
Add the noodles and bok choi to the pan and cook for 2 minutes or until tender and heated through. Ladle the soup into bowls. Arrange the chicken on top, then scatter over the bean sprouts, chilli and coriander and serve with lime wedges.