Gather all the ingredients.
Roll up your pork belly block and tie it with butcher twine.
Cut the green part of the Tokyo negi and set aside; reserve the white part for another use.
Cut 4 slices of ginger.
Heat 1 Tbsp neutral oil in a skillet over high heat and sear the pork belly until golden brown on all sides.
Prepare the braising liquid by combining sake, soy sauce, water, and sugar in a pot, then add the green part of the negi and ginger.
Transfer the seared pork belly to the pot and bring the liquid to a boil.
Once boiling, skim off the foam and turn the heat to low/simmer.
Cover with an otoshibuta and simmer for 2 hours, turning the chashu every 30 minutes.
After 2 hours, let the chashu cool slightly, then transfer it to a container or vacuum-sealing bag with some cooking liquid.
Refrigerate overnight to marinate.
The next day, slice the chashu into ¼-inch pieces and sear with a kitchen torch or broiler before serving.
