Wipe the pork belly dry with kitchen paper.
Lay the pork belly flat, skin side down. Sprinkle the meat with salt and pepper (optional step).
Roll the pork belly tie up with kitchen twine as tightly as you can.
Heat the oil in a wok and brown the rolled pork belly. For best results, turn the belly every few minutes to get even coloring.
In the slow cooker, stir together the soy sauce, sake, mirin and the bone broth, then drop in the browned pork belly.
Add the leeks and ginger.
Set the slow cooker on HIGH and cook the pork belly for five hours. For even coloring, AFTER the first two hours (the liquid will be bubbling by then), give the pork quarter turns every hour.
Cool the pork in the liquid.
Scoop out the pork, allow to drip and place in a container with a tight lid. Chill in the fridge overnight.
Take the pork out of the fridge and cut into slices.
Your chashu (Japanese braised pork belly) is now ready to go into your bowl of ramen. To make sure that the chashu is hot when served, when you ladle the hot broth into the bowl of ramen, ladle over the pork directly.