Step 1: Prepare the Pork Belly. Score the skin of the pork belly deeply in a crosshatch pattern, about 1/4 to 1/2 inch deep, being careful not to cut into the meat. Pat the pork belly thoroughly dry with paper towels. Season generously with salt and pepper.
Step 2: Blanch (Optional but Recommended). Place the scored pork belly in a large pot, cover with cold water, bring to a boil, then reduce heat and simmer for 10-15 minutes. Drain, rinse the pork, and pat it thoroughly dry again. This step helps tenderize the meat and render some fat.
Step 3: Initial Roast. Preheat your oven to 325°F (160°C). Place the pork belly, skin-side up, on a wire rack set over a baking sheet (lined with foil for easy cleanup). Roast for 1.5 to 2 hours, or until the fat has rendered significantly and the pork is very tender. The skin should start to crisp or become pliable.
Step 4: Prepare the Orange Glaze. While the pork is roasting, combine the orange juice, orange zest, soy sauce, honey (or brown sugar), rice vinegar, minced garlic, grated ginger, and red pepper flakes (if using) in a medium saucepan. Bring to a gentle boil over medium heat, then reduce heat and simmer for 10-15 minutes, or until the glaze thickens and becomes syrupy, coating the back of a spoon.
Step 5: Glaze and Caramelize. Increase the oven temperature to 400°F (200°C). Remove the pork belly from the oven. Brush a generous layer of the orange glaze over the pork belly. Return to the oven and bake for another 15-20 minutes, brushing with more glaze every 5-7 minutes, until the glaze is beautifully caramelized, sticky, and glossy. Watch closely to prevent burning.
Step 6: Rest and Serve. Once glazed to perfection, remove the pork belly from the oven and let it rest for 10-15 minutes before slicing. Resting allows the juices to redistribute, ensuring maximum tenderness. Slice into desired portions. Garnish with thinly sliced green onions and toasted sesame seeds. Serve hot with steamed rice and your favorite green vegetables.