Prep work: Place the ribs in a mixing bowl. Cover the ribs with cold water. Let it soak for 30 minutes.
Cut onion and spring onion into thin slices.
Make Braising Sauce: Mix in all of the listed ingredients and give a thorough whisk. Then place spring onions pieces in as well.
After soaking, drain the ribs and give another quick wash under running water. Then place the ribs in a large pot. Pour enough water to fully cover the ribs in the pot. Bring pot up to boil. Once it is boiling, place in ⅓ cup of mirin. This will get rid of any pork smell. Let it boil for 10 minutes.
After 10 minutes, strain it and give the ribs another rinse under running water.
Then place the rinsed ribs back into a pot. Add the onions strip on top of them. Then place the braising sauce in.
Place the pot on a medium heat and bring it up to a gentle simmer (adjust heat if needed). Let it reduce for about 50 minutes. We want to reduce the sauce until almost all of the liquid has evaporated. Remember to give it a stir once in a while.
After 50 minutes, most of the sauce should have evaporated. The leftover sauce will become thick and bubble-up, like runny maple syrup. This is the consistency you are looking for. Turn off the heat. It is ready.
Plate the ribs. Then get a scoop of the leftover sauce from the bottom of the top and spread over the ribs. Garnish with sesame seeds and spring onions. Enjoy!