To begin, trim the leftover lechon into manageable pieces, leaving skin and a bit of fat intact for flavour. Remove any burnt bits. Set aside while you prepare the sauce base.
Heat a large heavy based pot over medium heat. Add a little rendered fat or neutral oil. Sauté sliced onions until soft, then add crushed garlic. Stir gently until fragrant. Transition to adding liquids.
Pour in the pork stock and vinegar. Do not stir immediately. Let it simmer uncovered for 5 minutes to mellow the raw vinegar taste. Move to seasoning.
Add bay leaves, peppercorns, soy sauce and brown sugar. Stir until dissolved. This balance of sweet, salty and sharp flavours is essential to a well-rounded paksiw.
Lower the lechon pieces into the simmering liquid, skin side up if possible. Avoid stirring too much so the skin remains intact. Continue to the next step for slow simmering.
Cover the pot and reduce the heat to low. Simmer for 30–40 minutes until the pork is tender and has absorbed the sauce. Stir occasionally to prevent sticking but do so gently.
Add the Filipino liver sauce to the pot and mix it gently through. Simmer uncovered for another 10–15 minutes, allowing the sauce to thicken slightly. Taste and adjust salt.
If you enjoy a mild heat, drop in a whole red chilli and allow it to infuse in the final 5 minutes of simmering. Remove before serving if you prefer less heat.
Transfer the paksiw na lechon to a serving dish. Best enjoyed with steamed rice. Garnish with extra sauce from the pot. Presentation tip: Serve with a small side of pickled papaya (atchara) for a bright contrast.