Use a mandoline or cheese slicer to slice thin slices of the cucumber. Put them in a small bowl.
Heat the vinegar to dissolve the sugar in it (it doesn't necessarily need to boil for that).
Take the vinegar off the heat as soon as the sugar is dissolved and pour the vinegar of the cucumber slices and let it sit for a while.
Mise en place
Cut some small rings of the spring onion and chop some mint and cilantro.
Teriyaki mushrooms
Pull the oyster mushrooms lengthwise (along their natural lines) in small strips and cut the shiitake in quarters.
Heat some oil in a wok and stir fry the mushrooms for a few minutes until they have some color, then add the teriyaki sauce and let if heat for another few minutes.
Plating
Steam the buns as you prefer (microwave or otherwise).
Put some mushrooms, cucumbers, mint, cilantro, spring onions rings, crispy fried onions and kewpie on it and you're ready to serve!