Roll up the pork belly with the skin side out. Run some butcher twine under the middle of the pork. If your pork belly is long and wide, start under the far end instead of the middle.

Tie a double knot to secure the pork tightly. Leave the short end about 2 inches long. Pull the long end to wrap around the pork belly and tie another double knot.

In a pot of boiling water, blanch the pork belly for about 10 minutes. Transfer the pork to a pressure cooker. Add soy sauce, leek, ginger, water, sake, garlic, and mirin.

Cover with the lid and cook for 90 minutes on high pressure. Do a slow release and, after 20 minutes, open the lid. (Read Notes section).

Once the pork is cooled down, transfer the meat and sauce to a large sealed container or a zip-top bag. Put it in the refrigerator overnight or until it's completely cool. This way the pork is easier to slice thinly after cooling, and this also gives the pork more flavor.

When it's ready to serve, skim off the fatty oil on the surface. Transfer the pork to a cutting board and strain the sauce. Reserve the sauce for marinating soft-boiled eggs or for making more pork belly.

Take out the butcher twine. Cut the meat thinly using a very sharp knife. Reheat the slices in soup broth or heat it in a saucepan with the sauce until hot.