Pierce 700 g slab skinless pork belly on both sides with a fork, then roll it and secure it with string. Place in a pot and fill with cold water until submerged.
Bring to a boil on a medium-high setting and scoop out any scum that floats to the top.
Lower the heat to simmer and add 50 g Japanese leek (naganegi), 50 g ginger root, 3 cloves garlic, 100 g onion and 1 tsp rice vinegar. Place a drop lid on top and simmer on a low heat for 2 hours, turning half way through.
Remove the pork from the pot and place in a ziplock bag.
Transfer 250 ml pork stock from the chashu into a separate pan and add 50 ml sake, 1 tbsp mirin, 150 ml Japanese soy sauce (koikuchi shoyu) and 30 g sugar. Boil for 1 minute and then leave to cool.
Once cool to the touch, add the marinade to the ziplock bag along with 6 soft-boiled eggs (optional) and seal. Marinate for 12 hours (ideally overnight).
Heat a frying pan on medium-high and add the chashu. Add 100ml of leftover marinade and fry until thickened. Turn the pork occasionally to glaze all over.
Rest for 5-10 minutes before slicing. Typically the middle slices would be saved for ramen and the end cuts would be used for donburi.
Cut the white part of a Japanese leek (naganegi) halfway through horizontally and separate each layer. Press each layer flat on a chopping board and thinly slice. This is called "shiraganegi" and is used to garnish.
Soak the shiraganegi in a bowl of cold water for 5-10 minutes to remove any bitterness.
Dish up 6 portions cooked Japanese short-grain rice and cut the ramen eggs into halves. Evenly distribute the chashu and eggs between each portion and top with Japanese mayonnaise, drained shiraganegi and chili threads (optional).
Enjoy!
Notes
You can make this dish with leftover chashu instead of making it all from scratch.
The chashu can be kept in the fridge for up to 1 week, or 3-4 weeks in the freezer. (I recommend cutting it into slices before freezing.)