Cut the 600g pork belly into large cubes of about 3–4cm, keeping both fat and lean in each piece so they hold their shape through the long braise. Put the pork in a non-reactive bowl and add the 120ml soy sauce, 120ml vinegar, the 6 lightly crushed garlic cloves, 3 bay leaves and 1½ tsp whole black peppercorns, keeping the 2 sliced cloves aside for frying. Toss until every piece is coated, cover and refrigerate for 30 minutes so the pork stays cold. The marinade is optional but recommended; if you skip it, keep the liquids and aromatics together and add them all at the braising stage. Never discard the marinade, because it becomes the braising liquid. That short soak is what seasons the pork more evenly before the heat goes on.

While the pork marinates, put 1 tbsp of the neutral oil and the 2 thinly sliced garlic cloves in a small pan over low to medium-low heat. Fry slowly for 3–5 minutes, stirring often, until the slices turn an even pale gold and stop bubbling hard. Lift them out just before they look fully golden, since they keep darkening from the heat they carry, and drain on paper towel, where they crisp as they cool. They should be dry, crisp and light gold, never soft or deeply browned. If any slices turn dark brown, discard them, because they will taste bitter. Pour the garlic-scented oil into a small dish and keep it for finishing. That slow, low-heat frying is what dries the garlic into genuinely crisp chips before the sugars on the surface can scorch.

Lift the pork out of the marinade, letting the excess liquid drip back into the bowl, and reserve all of the marinade with its garlic, bay leaves and peppercorns. Pat the pork lightly with paper towel if it is very wet. Heat a heavy pot or deep skillet over medium heat and add the pork in a single layer, working in batches if needed. Brown it on several sides for 6–8 minutes in total, until the fat begins to render and the edges turn deep golden. If the pot looks dry at the start, add the remaining 1 tbsp neutral oil. Add the sliced onion, if using, and cook for 3–4 minutes until softened. Do not cook so long that the pork firms up. That careful browning is what builds the deeper, roasted flavour behind a genuinely dark adobo.

Pour the reserved marinade and the 180ml water over the pork, scraping up any browned bits from the base of the pot. Bring it up to a simmer over medium heat without stirring, letting the vinegar cook off its raw edge for a minute or two. Once simmering, stir gently, lower the heat and cook partially covered for 40–50 minutes, stirring occasionally. Keep it at a genuinely lazy simmer, with small bubbles breaking at the surface, never a hard boil. If the liquid drops below halfway up the pork before it is tender, add a splash more water. The pork is ready for the next stage when a fork slides in easily but each cube still holds its shape. That undisturbed simmer after the vinegar goes in is what keeps the sauce's sourness clean rather than harsh.

About 25 minutes before the braise is done, rinse the 300g rice in several changes of cold water, swirling it with your fingers, until the water runs mostly clear, then drain well. Put it in a medium saucepan with the 450–500ml water and the pinch of salt, if using, and bring to a boil over medium-high heat. Stir once, cover tightly, reduce the heat to low and cook for 12–15 minutes, until the water is absorbed. Take off the heat and rest, still covered, for 10 minutes while the sauce reduces, then fluff gently with a fork. The grains should be genuinely fluffy and separate, never wet or mushy. Keep the rice plainly seasoned. That covered rest is what lets the trapped steam finish the grains evenly so they can soak up the sauce without turning soft.

Remove the lid, raise the heat slightly to a steady simmer and cook for 15–20 minutes, turning the pork occasionally, until the sauce reduces to a glossy, dark brown, spoon-coating consistency. Taste it and adjust. If it tastes too sharp, stir in the optional 1 tsp brown sugar. If it tastes too salty, add a small splash of water and simmer briefly again. Do not reduce until dry; stop while there is still enough sauce to spoon over the rice and pool lightly in the bowl. The bay leaves and peppercorns stay in the sauce for the finished look. The adobo is done when the pork is fully tender, the fat is silky and the sauce clings to the meat with a lacquered sheen. That final uncovered reduction is what concentrates the sauce into a glaze without tightening the pork.

Divide the hot, fluffed rice among 4 bowls. Spoon the pork adobo over the rice, sharing the pieces evenly, then add a spoonful of the reduced sauce to each bowl so it runs lightly into the rice. Scatter over the 2 sliced scallions and the crispy garlic chips just before serving. Keep the presentation simple and home-style, with the glossy pork as the hero, white rice around it and the dark sauce clearly visible. Do not overload the bowl with garnish. Serve immediately, while the pork is hot and the garlic chips are still crisp. That restrained finish is what lets the rich, sharp sauce and the fresh scallions and crunchy garlic balance in every spoonful.