Slice the onion thin, cut the nira into 3 cm (1 in) lengths, and cut the kimchi into bite-size pieces. Have everything ready before you start cooking.
Cut the pork belly into bite-size pieces and season lightly with salt and pepper. Go light, because the kimchi is already salty.
Heat the sesame oil in a frying pan over medium heat until fragrant but not smoking.
Add the pork and onion together and stir-fry over medium heat until the pork has changed color all the way through and the onion is soft and translucent.
Add the kimchi with all of its brine and stir through for about a minute, until the liquid reduces slightly and coats the pork.
Add the nira and the mentsuyu and toss quickly for 30 seconds to 1 minute, just until the chives turn bright green. Serve immediately with hot white rice.