Determine which direction the grain of the pork belly runs by looking at the edges (the ends that look most like bacon are cut across the grain). Slice the pork belly ALONG the grain to create several logs that are about 2" wide along the "bacon" ends. The exact number of logs will depend on the shape of your original slab.
Heat the oil in a cast iron skillet or heavy frying pan over high heat. Place pork belly logs in the pan fat-side down and sear for about 1.5-2 minutes. Turn and repeat until all sides have been seared (about 10-12 minutes total time).
While meat is searing, add all remaining ingredients to a large stock pot, dutch oven, braising dish, or similar pan. Bring to a boil over high heat. Skim off any foam, then reduce heat to a simmer. Add the seared pork belly logs to the pot.
Make a circle of aluminum foil that is just wide enough to fit the INSIDE diameter of the pot (you want it to rest directly on top of contents of the pot while covering them completely). Cut a 1"-wide "X" in the center. (You have just made a makeshift otoshibuta, or Japanese-style vented drop lip. If you happen to own an actual otoshibuta, by all means use that instead).
Simmer on low heat for one hour, turning the pork belly every 15 minutes.
Remove the chashu from the pan and let rest. Skim fat from the remaining contents of the pot, then strain out solids. Reserve sauce for storing any chashu that is not going to be served immediately, or refrigerate for later use in other recipes.
If you are serving the chashu immediately, slice into ¼" slices across the grain, and serve however you prefer (i.e. atop a bowl of ramen or rice). Chashu can also be stored in the refrigerator for a few days prior to serving by leaving whole and placing in an airtight container or plastic bag with a small amount of sauce. For long-term storage, chashu may be sliced and frozen.