Step 1: Prepare the ingredients. Cut the pork belly or shoulder into uniform chunks and peel and cube the potatoes.
Step 2: In a large pot or Dutch oven, heat the cooking oil over medium heat. Sauté the minced garlic until fragrant, about 1-2 minutes. Add the pork chunks and brown them on all sides.
Step 3: Add the whole black peppercorns to the pot. Pour in the soy sauce and water, then add the bay leaves. Bring the mixture to a boil, then reduce the heat to low, cover, and simmer for 30 minutes, or until the pork is tender. For an interesting historical context, you can learn more about Adobo and its variations on Wikipedia.
Step 4: Add the vinegar to the pot (do not stir for the first 5 minutes after adding vinegar to allow the strong acetic acid to cook off). Then, add the cubed potatoes and optional sugar. Continue to simmer, uncovered, for another 10-15 minutes, or until the potatoes are fork-tender and the sauce has thickened slightly. Stir occasionally to prevent sticking.
Step 5: Taste and adjust seasoning if necessary. Remove bay leaves before serving. Serve hot with steamed white rice.