Add a tbsp of oil to a hot pan and heat pork belly strips until both sides are golden (about 5 minutes on each side)
Take pork belly out of the pan and carefully drain out the fat into a small bowl
Add pork belly back into the pan and add in onions and sauce. Simmer under low heat with lid slightly ajar until sauce reduces and thickens (at least one hour)
Slice up your chashu and enjoy over a bed of cooked white rice, ramen, etc. Be sure to ladle some of the chashu sauce over the meat before serving.