Preheat your oven to 300°F (150°C).
In a mixing bowl, whisk together the soy sauce, brown sugar, gochugaru, sesame oil, minced garlic, minced ginger, rice vinegar, and black pepper until smooth.
Place the pork belly in a large resealable bag or shallow dish and pour the marinade over it, ensuring it is thoroughly coated.
Seal the bag or cover the dish and marinate in the refrigerator for at least 2 hours, preferably overnight.
Arrange the marinated pork belly on a baking sheet lined with foil, skin side up, and reserve the marinade.
Bake in the oven for 2 to 2.5 hours, brushing with reserved marinade every half hour.
Let the pork belly rest for about 10 minutes before slicing into pieces.
Garnish with sesame seeds and chopped green onions before serving.