Heat oil in a pot. Add garlic and sauté until lightly browned. Add onions and cook until soft and fragrant.
Add the pork belly and cook until the edges start to brown and fat begins to render.
Pour in the soy sauce, peppercorn, and bay leaves. Cook for a few minutes so the pork absorbs flavor.
Pour in the water. Bring to a boil, then lower the heat. Cover and simmer for 35–45 minutes or until pork is tender.
Pour vinegar on top and let it boil untouched for 1–2 minutes. This prevents the raw sour taste.
Stir gently, add brown sugar if using, and simmer until the sauce thickens to your liking.
Add a bit more soy sauce for saltiness or water if the sauce reduces too much. Add dry chili if you want it spicy.
Serve and enjoy!