Cook the noodles in plenty of boiling water for about five minutes. Drain. Dump into a bowl of iced water and leave them there while you prepare the rest of the ingredients.
Blanch the carrot, sweet pea pods and oyster mushrooms for about a minute, dump into iced water then drain.
Heat the vegetable oil in a pan. Add the pork and cook, stirring, until no longer pink. Pour in the mirin. Cook, uncovered, until the liquid has evaporated and the pork starts to render fat, about seven minutes. The pork will caramelize in the residual sugar of the wine so watch closely so that they don’t get burnt.
As soon as the pork starts to brown, add the sliced onion and bok choy. Stir fry for about thirty seconds then pour in the miso paste mixture. Stir, scraping the bottom of the pan to loosen the delicious bits of caramelized goodness stuck there.
As soon as the mixture bubbles, turn off the heat. Add the carrot, pea pods and oyster mushrooms. Toss. Taste. Add salt and pepper, as needed.
Drain the noodles. Toss with the pork and vegetables. Sprinkle with toasted sesame seeds and serve the cha soba with miso sauce.