Marinate the pork: Place pork belly cubes in a large bowl and pour soy sauce over them. Mix well to ensure all pieces are coated. Cover and refrigerate for at least 1 hour, or overnight for deeper flavor.

Prepare the aromatics: Heat oil in a large pot or deep skillet over medium heat. Add minced garlic and sauté until golden brown, about 1-2 minutes. Add sliced onions and continue cooking until translucent and aromatic, about 3-4 minutes.

Cook the pork: Add the marinated pork (including the soy sauce marinade) to the pot. Sauté for 5 minutes, stirring occasionally until the pork begins to brown slightly.

Add remaining ingredients: Pour in the vinegar (DO NOT STIR immediately after adding vinegar - this is a Filipino cooking technique). Add bay leaves, black peppercorns, water, and brown sugar (if using). Let the mixture come to a boil without stirring.

Simmer: Once boiling, lower the heat, stir everything together, and cover. Simmer for about 30-35 minutes or until the pork is tender and the sauce has reduced.

Adjust seasoning: Taste and adjust with salt, pepper, or additional vinegar as needed. If you prefer a drier adobo, continue simmering uncovered until the sauce reaches your desired consistency.

Serve: Remove bay leaves and serve hot with steamed white rice.