This is a typical Pork Adobo (Adobong Baboy in tagalog) recipe, which is the signature dish of the Filipinos. Every household has their own favorite recipe for Pork Adobo that's served for lunch, dinner and even breakfast. It's also served at special occasions like fiestas, birthdays, Christmas, christenings, weddings, reunions and important gatherings. There are many different variations, but most include pork (belly or shoulder) in a mixture of garlic, vinegar and soy sauce. From there, recipes can vary a bit from region to region, both in ingredients used as well as how the pork is cooked.
Combine the soy sauce, garlic, sugar and peppercorns in a resealable gallon-size plastic bag. Add the pork, seal the bag closed, and marinate in the refrigerator for at least 2 hours, or overnight if possible (for flavors to meld). Mix every couple hours to marinate thoroughly.
Drain the pork in a colander over a bowl; reserve the marinade and garlic.
Heat the oil in a large Dutch oven over medium heat. Brown the pork in batches, making sure not to crowd the meat, and turn pork often until all sides are brown, 6 to 8 minutes per batch (it's OK if the peppercorns adhere to the pork). Watch the pork closely: The sugar in the marinade will cause the pork to darken quickly if the pot is too hot. Lower the heat if necessary, to avoid burning. Remove the pork with a slotted spoon; set aside.
Discard all but 2 tablespoons of the oil from the Dutch oven. Add the garlic from the marinade and the onions, and cook, stirring, until the onions are translucent, 10 to 12 minutes.
Return pork to Dutch oven. Add the strained marinade, water, and bay leaves, and bring to a boil. Lower the heat to medium, cover and cook at a medium boil, stirring periodically, until the pork is tender but not falling apart, about 1 hour and 25 minutes. The cooking liquid will be reduced by at least 2/3.
Add the vinegar, but do not stir. Cook, uncovered, until the sauce is reduced to the consistency of a loose marinara, about 20 minutes more. Remove from the heat, and skim and discard fat (pork belly will render a good amount). Allow the pork and sauce to sit 15 minutes before serving; the sauce will continue to thicken. Serve over jasmine rice.