Bring the stock to a gentle boil in a medium saucepan. Add the chicken breasts and simmer for 8 mins until cooked through. Remove from the pan and shred the meat.
Add the rice and carrots to the hot stock. Bring back up to the boil, cover with a lid, then reduce the heat and cook for 10 mins until the rice is cooked and the carrots are tender.
Return the chicken to the pan and add the miso, soy and mirin. Scatter over the spring onions just before serving.