In a pan over medium heat, add the pork belly and cook, turning as needed, for about 3 to 5 minutes or until browned and begins to render fat.
Add onions and garlic and cook until softened.
Add vinegar and bring to a boil, uncovered and without stirring, for about 2 to 3 minutes.
Add soy sauce and water. Bring to a boil, skimming any scum that floats on top.
Add bay leaves and peppercorns.
Lower heat, cover, and cook until meat is fork-tender and liquid is reduced.
Add oyster sauce and continue to cook for about 1 to 2 minutes.
Season with salt if needed. Serve hot.