In a bowl, mix together sugar, mirin, soy sauce, ginger and green onions.
Truss the pork belly with butchers twine, and put it in a freezer bag with the marinade. Remove the air through a vacuum sealer or the displacement method. Put the bag in the fridge, and let the pork marinade for 10 - 24 hours (optional, but recommended).
Set sous vide machine to 74C/165F. Once the target temperature is reached, drop pork belly in the bath for 10 hours.
Remove bag from bath, and immediately transfer bag to an ice bath to cool down the pork belly. Once cooled, take pork belly out and pat dry.
Heat a large skillet on high and add oil. Sear for 60 seconds on each side. Remove from pan, cut off the twine and place on plate to rest.
Slice the chashu into 1/4-inch slices. When ready to eat, sear both sides of the chashu on a hot pan for 1 minute a side, or until browned.