In a large casserole pan (with a lid), heat the oil over a medium/high heat until hot.
Add the pork in small batches and fry until browned. Repeat with all the pork then set aside in a bowl.
To the pan, add the onion, celery, carrot, garlic, bay leaves and peppercorns and fry for 2-3 minutes to soften.
Return the pork and juices to the pan along with the soy sauce, vinegar and salt. Bring to a boil and then add around 400ml water. Bring this to a boil then reduce the heat too low. Pop on the lid and simmer gently for 1 1/2 - 2 hours until the pork is tender - stirring occasionally.
Using a slotted spoon, remove as much pork as you can to a waiting bowl and then using a stick blender, blend the vegetables into the liquid to make a sauce. Return the pork, and you're done.
While the Adobo cooks, mix the onion and vinegar together to make the garnish. Refrigerate until needed.
Serve with boiled rice. Scatter over the pickled onions and sprinkle a little fresh coriander if you have any just before serving.