Clean & Poke the Skin: Rinse the pork belly under cold water. Use a knife to carefully scrape the skin to remove any impurities, then pat the entire cut of meat completely dry with paper towels. Using a sharp wooden skewer, poke holes evenly across the skin (be careful not to pierce through the fat layer into the meat).
Clean the Skin & Salt: Dip the corner of a paper towel into 1 Tablespoon of Shao Hsing wine and wipe it over the poked pork skin to clean it. Sprinkle the sea salt evenly across the surface of the skin.
Mix the Marinade: In a medium mixing bowl, stir together the diced yellow onion, minced garlic, brown sugar, five-spice powder, Shao Hsing wine, soy sauce, and ground white pepper (if using).
Build the Foil Boat & Marinate: Place the pork belly on a large sheet of aluminum foil. Wrap the foil up tightly around the sides of the meat to create a "boat," leaving the top skin completely exposed. Carefully pour the marinade down the sides of the foil boat (do not let it touch the skin) and gently massage the sides so it covers the meat.
Dry the Skin (Overnight): Place the foil boat into a baking dish. Put it in the refrigerator, completely uncovered, for at least 12 hours (24 hours is best for the crispiest result). The cold air will dry out the skin.
The First Roast (Low Heat): Preheat your oven to 300°F (150°C). Place the baking dish (with the pork still in its foil boat) on the lower-middle rack. Roast for 1 hour, or until the internal temperature reaches 135°F–145°F.
Blister the Skin (High Heat): Increase the oven temperature to 450°F (230°C). Roast the pork belly for an additional 30 - 45 minutes, or until the skin is bubbly, golden brown, and crispy.
Rest & Serve: Remove the pork from the oven and let it rest on a cutting board for 10 minutes. Slice and serve hot! Use Hoisin and Sriracha as your dipping sauces to add more depth of flavour.