Marinate the Pork:

In a bowl, combine the pork belly cubes, soy sauce, vinegar, minced garlic, bay leaves, black peppercorns, and sugar (if using). Mix well, ensuring the pork is well coated with the marinade. Let it marinate for at least 30 minutes to allow the flavors to infuse.

Cook the Pork Adobo:

Heat oil in a medium-sized pot over medium heat. Add the sliced onions and sauté for about 2 minutes until softened and aromatic. Add the marinated pork and all the marinade. Stir and cook for about 5 minutes, letting the pork brown slightly.

Add Water and Simmer:

Add 1/4 cup of water to the pot and stir. Lower the heat and let it simmer, covered, for about 30-40 minutes or until the pork is tender and the sauce has thickened. Stir occasionally and check if additional water is needed, adding more if necessary.

Thicken the Sauce (Optional):

If you prefer a thicker sauce, dissolve 1 tablespoon of cornstarch in 1 tablespoon of water to make a slurry. Add this mixture to the pot and stir well. Cook for another 2-3 minutes until the sauce reaches your desired consistency.

Season and Serve:

Taste the adobo and adjust seasoning with salt and pepper if needed. Once the pork is tender and the sauce has thickened, remove the pot from heat. Serve the Pork Adobo hot with steamed rice.