Remove the stems from the shiitake mushrooms and place them gill side up in a dish. In a small bowl mix together 2 tablespoons soy sauce, ½ tablespoon rice vinegar, the sugar and the sesame oil. Pour this marinade into the mushroom caps and leave to marinade while you prepare the veggies.
Heat a non-stick pan over medium high heat. Place the bok choy sliced side down and sear until just browned. Add a tablespoon or two of water to the pan and cover. Allow the bok choy to steam for a couple of minutes until bright green and the stems are tender.
Meanwhile, prepare the sauce by mixing together until smooth the remaining 1 ½ tablespoons soy sauce, 2 tablespoons rice vinegar, tahini, warm water, sambal, ginger and garlic.
Toss the mushrooms to coat in the marinade and pour out any residual water from the pan you used to cook the bok choy. Sear the mushrooms for a few minutes on both sides until brown and crispy. Pour a bit of the remaining marinade over them as you fry them.
Finally, divide the forbidden rice between two bowls and top with the vegetables. Garnish with some sliced green onions and sesame seeds, if desired, and serve with the sesame-ginger sauce.