Boil the defrosted chicken in plenty of lightly salted water for 2 minutes. Strain, rinse and discard the cooking water. Wash and peel the ginger and work it into small cubes. Wash, peel and cut the carrot into thin slices. Use only the green of the leek and cut across into thin strips.
Put the vegetables together with the chicken and the water in a casserole and simmer with the lid on for 45 minutes. Strain the broth and discard the material to be sieved.
In the meantime, cut the thawed chicken breast across the grain into approx. 3 mm thin slices and keep them ready. Wash the cucumber, cut off both ends and use an adjustable plane to plan off slices approx. 2 mm thick lengthways. Discard the first 2 slices. Cut the remaining slices crosswise into pieces approx. 4 cm long. Wash the spring onions and cut into thin rings from above.
Put the stock back in the Dutch oven and bring to a boil. Add the prepared chicken and the aji-no-moto and simmer for 1 minute. Add the cucumber slices and simmer for 2 minutes. Add the spring onions and sesame oil, remove the pan from the stove and distribute the soup hot on the serving bowls, serve and enjoy.