Cut pork belly into 4 inch lengths. Rub salt onto all sides of the pork belly and set aside.
Thinly slice ginger and jalapeno. Set aside.
Grind garlic with a grinder or a garlic press and set aside.
In a small mixing bowl, add water, Japanese sake, Soy Sauce, sweetener (start small and add more if needed), sliced ginger, sliced jalapeno and grated garlic. Mix well, pour sauce into a medium sized stove top pot and place on low heat.
In the meantime, add cooking oil into a cast iron pan and bring to smoking temperature. Once up to temperature, carefully add pork belly. Sear all 4 sides until nicely browned. Once seared, transfer to a holding plate.
Bring the sauce to a boil. Once boiling, add seared pork belly to pot. Reduce to low heat, cover with otoshibuta, and cook covered for 1 hour. Check every 15 minutes to flip chashu and ensure the sauce is not burning.
Slice green onions and set aside.
After 1 hour has passed, remove cover and raise heat to medium-low. Cook for an extra 10-15 minutes to reduce the sauce. Keep an eye on this to prevent. burning.
Once most of the sauce is reduced, remove from heat. Strain sauce.
Slice pork belly and transfer to serving plate. Garnish with green onions and serve! Optionally, broil or torch the slices before serving.