To make the broth, put the chicken and all the other ingredients (including the shitake with the soaking water), except the mushroom sauce into a big pot of water (about 2 1/2 litres) and bring up to the boil. Once on a rolling boil, reduce to a simmer and keep skimming off the scum rising to the surface till such time as the broth appears clear. Continue to slowly simmer for an hour.
After an hour, add the mushroom and soy sauces and check the seasoning. Take off the heat, remove the chicken pieces and mushrooms and set aside. Discard the ginger, garlic, onions, cinnamon and star anise.
Slice the chicken and discard the bones. Slice the shitake mushrooms thinly, discarding the stems. Set aside.
To make the rice, soak it in water for 20 minutes, drain and cook on a medium heat in 500 ml water, stirring occasionally. Add water in small quantities if required and cook till all the liquid is absorbed and the rice is cooked but not mushy.
Transfer the rice to a bowl and mix in the chillies, fish sauce, vinegar and mirin. Allow to cool before shaping into golf ball sized portions. Roll to coat with back sesame seeds and set aside.
To cook the mustard greens, blanch them in hot water for 15 seconds only and then quickly shock in ice water for a minute. Drain and roughly chop them for stir frying.
Heat oil in a pan till fairly hot but not smoking. Throw in the greens, minced garlic and chillies and fry for 2 minutes tossing frequently to make sure they don not burn. Season with salt. Cook for 30 seconds more and remove from the heat.
To assemble the meal, heat the broth. Place 3 rice balls in a bowl, add a generous helping of chicken and a few slices of mushroom, pour enough broth to partially cover the rice balls, sprinkle with chopped spring onions and serve immediately with the fried mustard greens on the side.