Heat the oil in a cast iron skillet (or regular frying pan) over high heat. Sear the fat side down first, then flip over to sear all sides, which will take about 10 minutes.
While searing, put all the ingredients for seasonings in a heavy-bottom pot (or regular pot) that fits the Chashu. Add the Chashu and bring it to a boil, skimming the scum and foam. Then turn the heat to low/simmer.
Put an Otoshibuta (drop lid) on top to press the ingredient down and limit the evaporation. If you don’t have an Otoshibuta, you can make it with aluminum foil. Simmer on low heat for next one hour, rotating Chashu every 15 minutes (keep Otoshibuta on all times!).
After one hour, there is ½ inch liquid left in the pot.
Transfer the Chashu to a container or a bag with a little bit of cooking sauce and refrigerate overnight. Strain the leftover cooking sauce and refrigerate (can use to make Ramen eggs).
To serve, slice the Chashu into ¼ inch pieces. You can use a propane torch or broiler to sear the Chashu slices to enhance the flavor.
If you kept the Chashu overnight and don't want to sear the Chashu, you can reheat it by soaking in the hot cooking sauce.