Preheat oven to 250˚F.
In a medium bowl, whisk together water, soy sauce, mirin and sugar until sugar dissolves. Set aside.
Heat oil over medium-high heat in a Dutch oven or other heavy, oven-safe pot until shimmering, but not quite smoking. Add pork belly and sear on all sides, about 2 minutes per side. Remove from heat.
Flip pork belly if needed so that the fat / skin side is down. Pour soy sauce mixture over (the liquid should come about half way up the side - add a bit more water if it doesn't) and add garlic, green onions and ginger to liquid. Cover pot and place in the center of preheated oven. Cook for 1 ½ hours; then flip pork belly to meat side and cook 1 hour more (see recipe note #2). Let pork belly cool in liquid, then transfer to a smaller container, pouring cooking liquid over the top; cover and refrigerate overnight.
The next day, scrape the fat off of the braising liquid and transfer pork to a cutting board. Strain solids from braising liquid and save for another use. (recipe note #3) With a sharp knife, cut into thin slices and pan fry until lightly golden in a non-stick skillet over medium heat. Serve as desired. (recipe note #4)
