Leave pork belly in room temperature for 30 minutes before cooking.
Salt & pepper over the pork belly. Cover with cling wrap and tenderize with a rolling pin or a meat mallet.
Roll the pork belly into a cylindrical shape.
Tie up the pork belly tightly with cooking string so the meat holds its shape (see the photo instruction in the post or watch the mini-movie above).
Heat the oil in a frying pan, brown the outside of the tied pork belly all around.
Transfer the pork belly into simmering water (not listed in the recipe. You need enough water to just cover the pork belly, (I used about 8 cups of water) and cook for one hour over low heat.
Drain the cooking water.
Add sugar, sake, soy sauce, water, scallions, garlic and ginger to the pot with the pork belly bring to boil then turn the heat down to low. Place a drop lid (otoshibuta) over the pork belly and simmer for one hour.
Skim scum off the cooking sauce at times and rotate the pork belly sometimes too.
After one hour, turn the heat off and leave the pork belly in the cooking sauce. *1
After the pork belly and the sauce cool down, refrigerate it overnight.
Take the pork belly out of the pot and cut the twine to remove carefully.
Slice the chashu 0.2 inch (5mm) wide and serve as ramen topping or store for later use. *2
Video
Notes
You can transfer to a container or leave it in the pot. I have enough space to leave the pot in the fridge, so left it in the pot.
If you are going to eat the Chashu as a dish, remove the fat and discard the scallions, garlic and ginger cooked with pork in the pot. Drain the sauce with a sieve and pour the liquid into a small saucepan. Bring it to boil then, reduce the heat down to a simmer. Continue to simmer the sauce until it is reduced and thickened. Pour this over the sliced Chashu and serve. Also see other suggestions to use the sauce in the above post.
Cling wrap the sliced chashu and place in a ziplock bag to store in freezer. It stores about 4 weeks in freezer and a week in fridge. I usually chop up the edge both ends of the rolled chashu to small bits and use them for "Chashu Don" or as fried rice or stir fry ingredients.
To store the cooking sauce, strain the cooking sauce with fine mesh sieve and discard scallions, garlic and ginger. It stores for a week in fridge and a month in freezer.